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1 CMSC 250 Chapter 4, con't., Inductive Proofs
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2 CMSC 250 Description l Inductive proofs must have: –Base case: where you prove that what it is you are trying to prove is true about the base case –Inductive hypothesis: where you state what will be assumed in the proof –Inductive step: show: –where you state what will be proven below proof: –where you prove what is stated in the show portion –this proof must use the inductive hypothesis somewhere
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3 CMSC 250 Example l Prove this statement: Base case ( n = 1): l Inductive hypothesis (assume the statement is true for n = p ): Inductive step (show the statement is true for n = p + 1), i.e, show :
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4 CMSC 250 Variations l 2 + 4 + 6 + 8 + … + 20 = ? l If you can, use the fact just proved, that: l Can it be rearranged into a form that works? l If not, it must be proved from scratch
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5 CMSC 250 Another example
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6 CMSC 250 Discrete Structures CMSC 250 Lecture 23 March 26, 2008
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7 CMSC 250 Another example- geometric progression
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8 CMSC 250 Another example- a divisibility property
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9 CMSC 250 A sequence example l Assume the following definition of a sequence: l Prove :
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10 CMSC 250 Discrete Structures CMSC 250 Lecture 24 March 28, 2008
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11 CMSC 250 An example with an inequality Prove this statement: Base case (n = 3): Inductive hypothesis (n = p): assume Inductive step (n = p + 1): Show:
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12 CMSC 250 Another example with an inequality
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13 CMSC 250 A less-mathematical example l If all we had was 2-cent coins and 5-cent coins, we could form any value greater than 3 cents. –Base case (n = 4): –Inductive hypothesis (n = p): –Inductive step (n = p + 1):
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14 CMSC 250 Discrete Structures CMSC 250 Lecture 25 March 31, 2008
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15 CMSC 250 Recurrence relation example l Assume the following definition of a function: l Prove the following definition property:
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16 CMSC 250 Strong induction l Regular induction: P(n) P(n+1) l With strong induction, the implication changes slightly: –if the statement to be proven is true for all preceding elements, then it's true for the current element ( n)[( i, a i n)[P(i)] P(n+1)] l The strong induction principle: P(0) … P(p) ( n)[P(0) P(1) P(2) … P(n) P(n + 1)] ( n ≥ 0)[P(n)]
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17 CMSC 250 Now prove the recurrence relation property, using strong induction l Here's the function definition again: l This is the property to be proven:
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18 CMSC 250 Another recurrence relation example l Assume the following definition of a function: l Prove the following definition property, using strong induction:
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19 CMSC 250 Discrete Structures CMSC 250 Lecture 26 April 2, 2008
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20 CMSC 250 Another example- a divisibility property l Assume the following definition of a recurrence relation: l Prove using strong induction that all elements in this relation have this property:
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21 CMSC 250 Another example l Assume the following definition of a recurrence relation: l Prove using strong induction that all elements in this relation have this property:
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22 CMSC 250 Discrete Structures CMSC 250 Lecture 27 April 4, 2008
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23 CMSC 250 Another example Theorem: for all n ≥ 2 there exist primes p 1, p 2,…, p k, and exponents e 1, e 2,… e k, such that
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24 CMSC 250 Constructive induction l Show: (i.e., find integers A and B for which this is true) In particular, we want to find the smallest A and B which will work
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25 CMSC 250 A factorial example
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