1. Quantum One

3. Ket-Bra Expansions and Integral Representations of Operators

4. In the last segment, we defined what we mean by Hermitian operators, anti- Hermitian operators, and unitary operators, and saw how any operator can be expressed in terms of its Hermitian and anti-Hermitian parts.
We then used the completeness relation for a discrete ONB to develop ket-bra expansions, and matrix representations of general linear operators, and saw how these matrix representations can be used to directly compute quantities related to the operators they represent.
Finally, we saw how to represent the matrix corresponding to the adjoint of an operator, and how Hermitian operators are represented by Hermitian matrices.
In this segment, we extend some of these ideas to continuously indexed bases sets, and develop integral representations of linear operators.

5. In the last segment, we defined what we mean by Hermitian operators, anti- Hermitian operators, and unitary operators, and saw how any operator can be expressed in terms of its Hermitian and anti-Hermitian parts.
We then used the completeness relation for a discrete ONB to develop ket-bra expansions, and matrix representations of general linear operators, and saw how these matrix representations can be used to directly compute quantities related to the operators they represent.
Finally, we saw how to represent the matrix corresponding to the adjoint of an operator, and how Hermitian operators are represented by Hermitian matrices.
In this segment, we extend some of these ideas to continuously indexed bases sets, and develop integral representations of linear operators.

6. In the last segment, we defined what we mean by Hermitian operators, anti- Hermitian operators, and unitary operators, and saw how any operator can be expressed in terms of its Hermitian and anti-Hermitian parts.
We then used the completeness relation for a discrete ONB to develop ket-bra expansions, and matrix representations of general linear operators, and saw how these matrix representations can be used to directly compute quantities related to the operators they represent.
Finally, we saw how to represent the matrix corresponding to the adjoint of an operator, and how Hermitian operators are represented by Hermitian matrices.
In this segment, we extend some of these ideas to continuously indexed bases sets, and develop integral representations of linear operators.

7. In the last segment, we defined what we mean by Hermitian operators, anti- Hermitian operators, and unitary operators, and saw how any operator can be expressed in terms of its Hermitian and anti-Hermitian parts.
We then used the completeness relation for a discrete ONB to develop ket-bra expansions, and matrix representations of general linear operators, and saw how these matrix representations can be used to directly compute quantities related to the operators they represent.
Finally, we saw how to represent the matrix corresponding to the adjoint of an operator, and how Hermitian operators are represented by Hermitian matrices.
In this segment, we extend some of these ideas to continuously indexed bases sets, and develop integral representations of linear operators.

8. Continuous Ket-Bra Expansion of Operators : Let form a continuous ONB for the space and let A be an operator acting in the space.
Then from the trivial identity
we can write

9. Continuous Ket-Bra Expansion of Operators : Let form a continuous ONB for the space and let A be an operator acting in the space.
Then from the trivial identity
we can write

10. Continuous Ket-Bra Expansion of Operators : Let form a continuous ONB for the space and let A be an operator acting in the space.
Then from the trivial identity
we can write

11. Continuous Ket-Bra Expansion of Operators : Let form a continuous ONB for the space and let A be an operator acting in the space.
Then from the trivial identity
we can write

12. Continuous Ket-Bra Expansion of Operators : Let form a continuous ONB for the space and let A be an operator acting in the space.
Then from the trivial identity
we can write

13. Continuous Ket-Bra Expansion of Operators : Let form a continuous ONB for the space and let A be an operator acting in the space.
Then from the trivial identity
we can write

14. Continuous Ket-Bra Expansion of Operators : Let form a continuous ONB for the space and let A be an operator acting in the space.
Then from the trivial identity
we can write

15. Continuous Ket-Bra Expansion of Operators : Let form a continuous ONB for the space and let A be an operator acting in the space.
Then from the trivial identity
we can write
This gives what we call a ket-bra expansion for this operator in this representation, and completely specifies the linear operator A in terms of its matrix elements
taken between the basis states of this representation.

16. Integral Representation of Operators
Thus in the wave function representation induced by any continuous ONB, an operator A is naturally represented by an integral kernel,
which is a function of two continuous indices, or arguments, the values of which that are just the matrix elements of A connecting the different members of the basis states defining that continuous representation.
Like the matrices associated with discrete representations, knowledge of the kernel facilitates computing quantities related to A itself.

17. Integral Representation of Operators
Thus in the wave function representation induced by any continuous ONB, an operator A is naturally represented by an integral kernel,
which is a function of two continuous indices, or arguments, the values of which that are just the matrix elements of A connecting the different members of the basis states defining that continuous representation.
Like the matrices associated with discrete representations, knowledge of the kernel facilitates computing quantities related to A itself.

18. Integral Representation of Operators
Thus in the wave function representation induced by any continuous ONB, an operator A is naturally represented by an integral kernel,
which is a function of two continuous indices, or arguments, the values of which are just the matrix elements of A connecting the different members of the basis states defining that continuous representation.
Like the matrices associated with discrete representations, knowledge of the kernel facilitates computing quantities related to A itself.

19. Integral Representation of Operators
Thus in the wave function representation induced by any continuous ONB, an operator A is naturally represented by an integral kernel,
which is a function of two continuous indices, or arguments, the values of which are just the matrix elements of A connecting the different members of the basis states defining that continuous representation.
Like the matrices associated with discrete representations, knowledge of the kernel facilitates computing quantities related to A itself.

20. Integral Representation of Operators
Thus, suppose that
for some states and
The expansion coefficients for the states and are then clearly related.
Note that if
then
which can be written, rather like a continuous matrix operation

21. Integral Representation of Operators
Thus, suppose that
for some states and
The expansion coefficients for the states and are then clearly related.
Note that if
then
which can be written, rather like a continuous matrix operation

22. Integral Representation of Operators
Thus, suppose that
for some states and
The expansion coefficients for the states and are then clearly related.
Note that if
then
which can be written, rather like a continuous matrix operation

23. Integral Representation of Operators
Thus, suppose that
for some states and
The expansion coefficients for the states and are then clearly related.
Note that if
then
which can be written, rather like a continuous matrix operation

24. Integral Representation of Operators
Thus, suppose that
for some states and
The expansion coefficients for the states and are then clearly related.
Note that if
then
which can be written, rather like a continuous matrix operation

25. Integral Representation of Operators
Thus, suppose that
for some states and
The expansion coefficients for the states and are then clearly related.
Note that if
then
which can be written, rather like a continuous matrix operation

26. Integral Representation of Operators
Thus, suppose that
for some states and
The expansion coefficients for the states and are then clearly related.
Note that if
then
which can be written, rather like a continuous matrix operation

27. Integral Representation of Operators
Consider the matrix element of A between arbitrary states and
Inserting our expansion for A this becomes
where identifying the wave functions for the two states involved we can write
which is the continuous version of product of a row-vector, a square matrix, and a column-vector.

28. Integral Representation of Operators
Consider the matrix element of A between arbitrary states and
Inserting our expansion for A this becomes
where identifying the wave functions for the two states involved we can write
which is the continuous version of product of a row-vector, a square matrix, and a column-vector.

29. Integral Representation of Operators
Consider the matrix element of A between arbitrary states and
Inserting our expansion for A this becomes
where identifying the wave functions for the two states involved we can write
which is the continuous version of product of a row-vector, a square matrix, and a column-vector.

30. Integral Representation of Operators
Consider the matrix element of A between arbitrary states and
Inserting our expansion for A this becomes
where identifying the wave functions for the two states involved we can write
which is the continuous version of product of a row-vector, a square matrix, and a column-vector.

31. Integral Representation of Operators As another example, consider the operator product of
and
The operator product has a similar expansion, i.e.,
where
which gives the continuous analog of a matrix multiplication, i.e.,

32. Integral Representation of Operators As another example, consider the operator product of
and
The operator product has a similar expansion, i.e.,
where
which gives the continuous analog of a matrix multiplication, i.e.,

33. Integral Representation of Operators As another example, consider the operator product of
and
The operator product has a similar expansion, i.e.,
where
which gives the continuous analog of a matrix multiplication, i.e.,

34. Integral Representation of Operators As another example, consider the operator product of
and
The product operator has a similar expansion, i.e.,
where
which gives the continuous analog of a matrix multiplication, i.e.,

35. Integral Representation of Operators As another example, consider the operator product of
and
The product operator has a similar expansion, i.e.,
where
which gives the continuous analog of a matrix multiplication, i.e.,

36. Integral Representation of Operators As another example, consider the operator product of
and
The product operator has a similar expansion, i.e.,
where
which gives the continuous analog of a matrix multiplication, i.e.,

37. Integral Representation of Operators As another example, consider the operator product of
and
The product operator has a similar expansion, i.e.,
where
which gives the continuous analog of a matrix multiplication, i.e.,

38. Integral Representation of Operators As another example, consider the operator product of
and
The product operator has a similar expansion, i.e.,
where
which gives the continuous analog of a matrix multiplication, i.e.,

39. Integral Representation of Operators As another example, consider the operator product of
and
The product operator has a similar expansion, i.e.,
where
which gives the continuous analog of a matrix multiplication, i.e.,

40. Integral Representation of Operators As another example, consider the operator product of
and
The product operator has a similar expansion, i.e.,
where
which gives the continuous analog of a matrix multiplication, i.e.,

41. Integral Representation of Operators As another example, consider the operator product of
and
The product operator has a similar expansion, i.e.,
where
which gives the continuous analog of a matrix multiplication, i.e.,

42. Integral Representation of Operators As another example, consider the operator product of
and
The product operator has a similar expansion, i.e.,
where
which gives the continuous analog of a matrix multiplication, i.e.,

43. Integral Representation of Operators So if we know the kernels and
representing and , we can compute the kernel representing C = AB through the integral relation

44. Integral Representation of Operators: As a final example, consider the integral kernel representing the adjoint of an operator. If
then by the two-part rule we developed for taking the adjoint, it follows that
We can now switch the prime on the integration variables, and reorder, to find that
from which we deduce that

45. Integral Representation of Operators: As a final example, consider the integral kernel representing the adjoint of an operator. If
then by the two-part rule we developed for taking the adjoint, it follows that
We can now switch the prime on the integration variables, and reorder, to find that
from which we deduce that

46. Integral Representation of Operators: As a final example, consider the integral kernel representing the adjoint of an operator. If
then by the two-part rule we developed for taking the adjoint, it follows that
We can now switch the prime on the integration variables, and reorder, to find that
from which we deduce that

47. Integral Representation of Operators: As a final example, consider the integral kernel representing the adjoint of an operator. If
then by the two-part rule we developed for taking the adjoint, it follows that
We can now switch the prime on the integration variables, and reorder, to find that
from which we deduce that

48. Integral Representation of Operators: As a final example, consider the integral kernel representing the adjoint of an operator. If
then by the two-part rule we developed for taking the adjoint, it follows that
We can now switch the prime on the integration variables, and reorder, to find that
from which we deduce that

49. Integral Representation of Operators: As a final example, consider the integral kernel representing the adjoint of an operator. If
then by the two-part rule we developed for taking the adjoint, it follows that
We can now switch the prime on the integration variables, and reorder, to find that
from which we deduce that

50. This, obviously, is just the continuous analog of the complex-conjugate transpose of a matrix
A Hermitian operator is equal to its adjoint, so that the integral kernels representing Hermitian operators obey the relation

51. This, obviously, is just the continuous analog of the complex-conjugate transpose of a matrix
A Hermitian operator is equal to its adjoint, so that the integral kernels representing Hermitian operators obey the relation

52. Examples: As an example, in 3D, the operator has as its matrix elements in the position representation
This allows us to construct the expansion for this operator
where the double integral has been reduced to a single integral because of the delta function.
The operator is said to be diagonal in the position representation, because it has no nonzero elements connecting different basis states.

53. Examples: As an example, in 3D, the operator has as its matrix elements in the position representation
This allows us to construct the expansion for this operator
where the double integral has been reduced to a single integral because of the delta function.
The operator is said to be diagonal in the position representation, because it has no nonzero elements connecting different basis states.

54. Examples: As an example, in 3D, the operator has as its matrix elements in the position representation
This allows us to construct the expansion for this operator
where the double integral has been reduced to a single integral because of the delta function.
The operator is said to be diagonal in the position representation, because it has no nonzero elements connecting different basis states.

55. Examples: As an example, in 3D, the operator has as its matrix elements in the position representation
This allows us to construct the expansion for this operator
where the double integral has been reduced to a single integral because of the delta function.
The operator is said to be diagonal in the position representation, because it has no nonzero elements connecting different basis states.

56. Examples: As an example, in 3D, the operator has as its matrix elements in the position representation
This allows us to construct the expansion for this operator
where the double integral has been reduced to a single integral because of the delta function.
The operator is said to be diagonal in the position representation, because it has no nonzero elements connecting different basis states.

57. Examples: As an example, in 3D, the operator has as its matrix elements in the position representation
This allows us to construct the expansion for this operator
where the double integral has been reduced to a single integral because of the delta function.
The operator is said to be diagonal in the position representation, because it has no nonzero elements connecting different basis states.

58. Examples: As an example, in 3D, the operator has as its matrix elements in the position representation
This allows us to construct the expansion for this operator
where the double integral has been reduced to a single integral because of the delta function.
The operator is said to be diagonal in the position representation, because it has no nonzero elements connecting different basis states.

59. Examples: As an example, in 3D, the operator has as its matrix elements in the position representation
This allows us to construct the expansion for this operator
where the double integral has been reduced to a single integral because of the delta function.
The operator is said to be diagonal in the position representation, because it has no nonzero elements connecting different basis states.

60. Examples: As an example, in 3D, the operator has as its matrix elements in the position representation
This allows us to construct the expansion for this operator
where the double integral has been reduced to a single integral because of the delta function. Thus,
The operator is said to be diagonal in the position representation, because it has no nonzero elements connecting different basis states.

61. Examples: As an example, in 3D, the operator has as its matrix elements in the position representation
This allows us to construct the expansion for this operator
where the double integral has been reduced to a single integral because of the delta function. Thus,
The operator is said to be diagonal in the position representation, because it has no nonzero elements connecting different basis states.

62. This concept of diagonality extends to arbitrary representations.
An operator A is said to be diagonal in the discrete representation if
so that
which only has one summation index,
in contrast to the general form which requires two.

63. This concept of diagonality extends to arbitrary representations.
An operator A is said to be diagonal in the discrete representation if
so that
which only has one summation index,
in contrast to the general form which requires two.

64. This concept of diagonality extends to arbitrary representations.
An operator A is said to be diagonal in the discrete representation if
so that
which only has one summation index,
in contrast to the general form which requires two.

65. This concept of diagonality extends to arbitrary representations.
An operator A is said to be diagonal in the discrete representation if
so that
which only has one summation index,
in contrast to the general form which requires two.

66. This concept of diagonality extends to arbitrary representations.
An operator A is said to be diagonal in the discrete representation if
so that
which only has one summation index,
in contrast to the general form which requires two.

67. This concept of diagonality extends to arbitrary representations.
An operator A is said to be diagonal in the discrete representation if
so that
which only has one summation index,
in contrast to the general form which requires two.

68. This concept of diagonality extends to arbitrary representations.
An operator A is said to be diagonal in the discrete representation if
so that
which only has one summation index,
in contrast to the general form which requires two.

69. In a discrete representation, an operator that is diagonal in that representation is represented by a diagonal matrix, i.e., if
then

70. In a discrete representation, an operator that is diagonal in that representation is represented by a diagonal matrix, i.e., if
then

71. In a discrete representation, an operator that is diagonal in that representation is represented by a diagonal matrix, i.e., if
then

72. Similarly, in a continuous representation an operator A is diagonal if
so that
which ends up with only one integration variable, i.e.,
in contrast to the general form which requires two.

73. Similarly, in a continuous representation an operator A is diagonal if
so that
which ends up with only one integration variable, i.e.,
in contrast to the general form which requires two.

74. Similarly, in a continuous representation an operator A is diagonal if
so that
which ends up with only one integration variable, i.e.,
in contrast to the general form which requires two.

75. Similarly, in a continuous representation an operator A is diagonal if
so that
which ends up with only one integration variable, i.e.,
in contrast to the general form which requires two.

76. Similarly, in a continuous representation an operator A is diagonal if
so that
which ends up with only one integration variable, i.e.,
in contrast to the general form which requires two.

77. It is easy to show that in any basis in which an operator is diagonal, it is what we referred to earlier as a multiplicative operator in that representation.
That is, if
is diagonal in the representation, and if
then
which shows that a diagonal operator G acts in the representation to multiply the wave function by

78. It is easy to show that in any basis in which an operator is diagonal, it is what we referred to earlier as a multiplicative operator in that representation.
That is, if
is diagonal in the representation, and if
then
which shows that a diagonal operator G acts in the representation to multiply the wave function by

79. It is easy to show that in any basis in which an operator is diagonal, it is what we referred to earlier as a multiplicative operator in that representation.
That is, if
is diagonal in the representation, and if
then
which shows that a diagonal operator G acts in the representation to multiply the wave function by

80. It is easy to show that in any basis in which an operator is diagonal, it is what we referred to earlier as a multiplicative operator in that representation.
That is, if
is diagonal in the representation, and if
then
which shows that a diagonal operator G acts in the representation to multiply the wave function by

81. It is easy to show that in any basis in which an operator is diagonal, it is what we referred to earlier as a multiplicative operator in that representation.
That is, if
is diagonal in the representation, and if
then
which shows that a diagonal operator G acts in the representation to multiply the wave function by

82. It is easy to show that in any basis in which an operator is diagonal, it is what we referred to earlier as a multiplicative operator in that representation.
That is, if
is diagonal in the representation, and if
then
which shows that a diagonal operator G acts in the representation to multiply the wave function by

83. We list below ket-bra expansions and matrix elements of important operators.
The position operator
The potential energy operator
The wavevector operator
The momentum operator
The kinetic energy operator

84. We list below ket-bra expansions and matrix elements of important operators.
The position operator
The potential energy operator
The wavevector operator
The momentum operator
The kinetic energy operator

85. We list below ket-bra expansions and matrix elements of important operators.
The position operator
The potential energy operator
The wavevector operator
The momentum operator
The kinetic energy operator

86. We list below ket-bra expansions and matrix elements of important operators.
The position operator
The potential energy operator
The wavevector operator
The momentum operator
The kinetic energy operator

87. We list below ket-bra expansions and matrix elements of important operators.
The position operator
The potential energy operator
The wavevector operator
The momentum operator
The kinetic energy operator

88. Another nice thing about ket-bra expansions of this sort, particularly in a representation in which the operator is diagonal, is that it is very easy to determine whether or not the operator is Hermitian.
For example, we can take the Hermitian adjoint of the position operator
by replacing each term in this continuous summation with its adjoint.
Thus we easily see that
so the position operator (and each of its components) is clearly Hermitian.

89. Another nice thing about ket-bra expansions of this sort, particularly in a representation in which the operator is diagonal, is that it is very easy to determine whether or not the operator is Hermitian.
For example, we can take the Hermitian adjoint of the position operator
by replacing each term in this continuous summation with its adjoint.
Thus we easily see that
so the position operator (and each of its components) is clearly Hermitian.

90. Another nice thing about ket-bra expansions of this sort, particularly in a representation in which the operator is diagonal, is that it is very easy to determine whether or not the operator is Hermitian.
For example, we can take the Hermitian adjoint of the position operator
by replacing each term in this continuous summation with its adjoint.
Thus we easily see that
so the position operator (and each of its components) is clearly Hermitian.

91. Another nice thing about ket-bra expansions of this sort, particularly in a representation in which the operator is diagonal, is that it is very easy to determine whether or not the operator is Hermitian.
For example, we can take the Hermitian adjoint of the position operator
by replacing each term in this continuous summation with its adjoint. Thus we easily see that
so the position operator (and each of its components) is clearly Hermitian.

92. Another nice thing about ket-bra expansions of this sort, particularly in a representation in which the operator is diagonal, is that it is very easy to determine whether or not the operator is Hermitian.
For example, we can take the Hermitian adjoint of the position operator
by replacing each term in this continuous summation with its adjoint. Thus we easily see that
so the position operator (and each of its components) is clearly Hermitian.

93. Another nice thing about ket-bra expansions of this sort, particularly in a representation in which the operator is diagonal, is that it is very easy to determine whether or not the operator is Hermitian.
For example, we can take the Hermitian adjoint of the position operator
by replacing each term in this continuous summation with its adjoint. Thus we easily see that
so the position operator (and each of its components) is clearly Hermitian.

94. Another nice thing about ket-bra expansions of this sort, particularly in a representation in which the operator is diagonal, is that it is very easy to determine whether or not the operator is Hermitian.
For example, we can take the Hermitian adjoint of the position operator
by replacing each term in this continuous summation with its adjoint.
Thus we easily see that
so the position operator (and each of its components) is clearly Hermitian.

95. You should verify to yourself that each of the basic operators whose diagonal ket- bra expansion we previously displayed is also Hermitian.
It follows that the wavevector operator is also Hermitian,
and so the operator D=iK,
satisfies the relation D⁺=-iK⁺=-iK=-D
Thus we see that the operator D, which takes the gradient in the position representation, is actually an anti-Hermitian operator,
That’s why we traded it in for the wavevector operator.

96. You should verify to yourself that each of the basic operators whose diagonal ket- bra expansion we previously displayed is also Hermitian.
It follows that the wavevector operator is Hermitian,
and so the operator D=iK,
satisfies the relation D⁺=-iK⁺=-iK=-D
Thus we see that the operator D, which takes the gradient in the position representation, is actually an anti-Hermitian operator,
That’s why we traded it in for the wavevector operator.

97. You should verify to yourself that each of the basic operators whose diagonal ket- bra expansion we previously displayed is also Hermitian.
It follows that the wavevector operator is Hermitian,
and so the operator D=iK,
satisfies the relation D⁺=-iK⁺=-iK=-D
Thus we see that the operator D, which takes the gradient in the position representation, is actually an anti-Hermitian operator,
That’s why we traded it in for the wavevector operator.

98. You should verify to yourself that each of the basic operators whose diagonal ket- bra expansion we previously displayed is also Hermitian.
It follows that the wavevector operator is Hermitian,
and so the operator D=iK,
satisfies the relation D⁺=-iK⁺=-iK=-D
Thus we see that the operator D, which takes the gradient in the position representation, is actually an anti-Hermitian operator,
That’s why we traded it in for the wavevector operator.

99. You should verify to yourself that each of the basic operators whose diagonal ket- bra expansion we previously displayed is also Hermitian.
It follows that the wavevector operator is Hermitian,
and so the operator D=iK,
satisfies the relation D⁺=-iK⁺=-iK=-D
Thus we see that the operator D, which takes the gradient in the position representation, is actually an anti-Hermitian operator,
That’s why we traded it in for the wavevector operator.

100. You should verify to yourself that each of the basic operators whose diagonal ket- bra expansion we previously displayed is also Hermitian.
It follows that the wavevector operator is Hermitian,
and so the operator D=iK,
satisfies the relation D⁺=-iK⁺=-iK=-D
Thus we see that the operator D, which takes the gradient in the position representation, is actually an anti-Hermitian operator,
That’s why we traded it in for the wavevector operator.

101. You should verify to yourself that each of the basic operators whose diagonal ket- bra expansion we previously displayed is also Hermitian.
It follows that the wavevector operator is Hermitian,
and so the operator D=iK,
satisfies the relation D⁺=-iK⁺=-iK=-D
Thus we see that the operator D, which takes the gradient in the position representation, is actually an anti-Hermitian operator,
That’s why we traded it in for the wavevector operator.

102. You should verify to yourself that each of the basic operators whose diagonal ket- bra expansion we previously displayed is also Hermitian.
It follows that the wavevector operator is Hermitian,
and so the operator D=iK,
satisfies the relation D⁺=-iK⁺=-iK=-D
Thus we see that the operator D, which takes the gradient in the position representation, is actually an anti-Hermitian operator,
That’s why we traded it in for the wavevector operator.

103. As an additional example, we work out below the matrix elements of the wavevector operator K in the position representation.
Recall that for any state |ψ〉 for which the state has a position wave function given by the expression
But we we can also write
where
The right hand side of this last expression seems to be the wave function in the position representation for the state

104. As an additional example, we work out below the matrix elements of the wavevector operator K in the position representation.
Recall that for any state |ψ〉 for which the state has a position wave function given by the expression
But we we can also write
where
The right hand side of this last expression seems to be the wave function in the position representation for the state

105. As an additional example, we work out below the matrix elements of the wavevector operator K in the position representation.
Recall that for any state |ψ〉 for which the state has a position wave function given by the expression
But we we can also write
where
The right hand side of this last expression seems to be the wave function in the position representation for the state

106. As an additional example, we work out below the matrix elements of the wavevector operator K in the position representation.
Recall that for any state |ψ〉 for which the state has a position wave function given by the expression
But we we can also write
where
The right hand side of this last expression seems to be the wave function in the position representation for the state

107. As an additional example, we work out below the matrix elements of the wavevector operator K in the position representation.
Recall that for any state |ψ〉 for which the state has a position wave function given by the expression
But we we can also write
where
The right hand side of this last expression seems to be the wave function in the position representation for the state

108. Reminding ourselves of the position eigenfunctions
we see that, evidently
i.e.,
is -i times the gradient of the delta function.
The properties of this not-so-frequently encountered object are reviewed in the appendix at the end of the first chapter. The most important of which is that for any function ,

109. Reminding ourselves of the position eigenfunctions
we see that, evidently
is -i times the gradient of the delta function.
The properties of this not-so-frequently encountered object are reviewed in the appendix at the end of the first chapter. The most important of which is that for any function ,

110. Reminding ourselves of the position eigenfunctions
we see that, evidently
Is times the gradient of the delta function.
The properties of this not-so-frequently encountered object are reviewed in the appendix at the end of the first chapter. The most important of which is that for any function ,

111. Reminding ourselves of the position eigenfunctions
we see that, evidently
is times the gradient of the delta function.
The properties of this not-so-frequently encountered object are reviewed in the appendix at the end of the first chapter. The most important of which is that for any function ,

112. Reminding ourselves of the position eigenfunctions
we see that, evidently
is times the gradient of the delta function.
The properties of this not-so-frequently encountered object are reviewed in the appendix at the end of the first chapter. The most important of which is that for any function ,

113. Reminding ourselves of the position eigenfunctions
we see that, evidently
is times the gradient of the delta function.
The properties of this not-so-frequently encountered object are reviewed in the appendix at the end of the first chapter. The most important of which is that for any function ,

114. Reminding ourselves of the position eigenfunctions
we see that, evidently
is times the gradient of the delta function.
The properties of this not-so-frequently encountered object are reviewed in the appendix at the end of the first chapter. The most important of which is that for any function ,

115. We deduce, therefore that can be expanded in the position representation in the form
so that when we apply this to any state , we obtain
consistent with our previous definition.

116. We deduce, therefore that can be expanded in the position representation in the form
so that when we apply this to any state , we obtain
consistent with our previous definition.

117. We deduce, therefore that can be expanded in the position representation in the form
so that when we apply this to any state , we obtain
consistent with our previous definition.

118. We deduce, therefore that can be expanded in the position representation in the form
so that when we apply this to any state , we obtain
consistent with our previous definition.

119. We deduce, therefore that can be expanded in the position representation in the form
so that when we apply this to any state , we obtain
consistent with our original definition.

120. In a similar fashion, one finds that
and that

121. In a similar fashion, one finds that
and that

122. In this segment, we used the completeness relation for continuous ONBs to develop ket-bra expansions, and integral representations of linear operators.
We then saw how the integral kernel associated with these representations can be used to directly compute quantities related to the operators they represent.
We also introduced the notion of diagonality of an operator in a given representation, and developed expansions for the basic operators of a single particle in representations in which they are diagonal.
Finally, we saw how differential operators can be expressed as ket-bra expansions with integral kernels that involve derivatives the delta function, so that we could understand how a linear operator acting on kets, can somehow end up replacing the wave function with its derivative or gradient.
In the next lecture we consider still other, representation independent, properties of linear operators.

123. In this segment, we used the completeness relation for continuous ONBs to develop ket-bra expansions, and integral representations of linear operators.
We then saw how the integral kernel associated with these representations can be used to directly compute quantities related to the operators they represent.
We also introduced the notion of diagonality of an operator in a given representation, and developed expansions for the basic operators of a single particle in representations in which they are diagonal.
Finally, we saw how differential operators can be expressed as ket-bra expansions with integral kernels that involve derivatives the delta function, so that we could understand how a linear operator acting on kets, can somehow end up replacing the wave function with its derivative or gradient.
In the next lecture we consider still other, representation independent, properties of linear operators.

124. In this segment, we used the completeness relation for continuous ONBs to develop ket-bra expansions, and integral representations of linear operators.
We then saw how the integral kernel associated with these representations can be used to directly compute quantities related to the operators they represent.
We also introduced the notion of diagonality of an operator in a given representation, and developed expansions for the basic operators of a single particle in representations in which they are diagonal.
Finally, we saw how differential operators can be expressed as ket-bra expansions with integral kernels that involve derivatives the delta function, so that we could understand how a linear operator acting on kets, can somehow end up replacing the wave function with its derivative or gradient.
In the next lecture we consider still other, representation independent, properties of linear operators.

125. In this segment, we used the completeness relation for continuous ONBs to develop ket-bra expansions, and integral representations of linear operators.
We then saw how the integral kernel associated with these representations can be used to directly compute quantities related to the operators they represent.
We also introduced the notion of diagonality of an operator in a given representation, and developed expansions for the basic operators of a single particle in representations in which they are diagonal.
Finally, we saw how differential operators can be expressed as ket-bra expansions with integral kernels that involve derivatives the delta function, so that we could understand how a linear operator acting on kets, can somehow end up replacing the wave function with its derivative or gradient.
In the next lecture we consider still other, representation independent, properties of linear operators.

126. In this segment, we used the completeness relation for continuous ONBs to develop ket-bra expansions, and integral representations of linear operators.
We then saw how the integral kernel associated with these representations can be used to directly compute quantities related to the operators they represent.
We also introduced the notion of diagonality of an operator in a given representation, and developed expansions for the basic operators of a single particle in representations in which they are diagonal.
Finally, we saw how differential operators can be expressed as ket-bra expansions with integral kernels that involve derivatives the delta function, so that we could understand how a linear operator acting on kets, can somehow end up replacing the wave function with its derivative or gradient.
In the next segment we consider still other, representation independent, properties of linear operators.