1. MATH 3286 Mathematics of Finance
Instructor:
Dr. Alexandre Karassev

2. COURSE OUTLINE Theory of Interest Life Insurance
Interest: the basic theory
Interest: basic applications
Annuities
Amortization and sinking funds
Bonds
Life Insurance
Preparation for life contingencies
Life tables and population problems
Life annuities
Life insurance

3. Chapter 1 INTEREST: THE BASIC THEORY
Accumulation Function
Simple Interest
Compound Interest
Present Value and Discount
Nominal Rate of Interest
Force of Interest

4. 1.1 ACCUMULATION FUNCTION
Definitions
The amount of money initially invested is called the principal.
The amount of money principal has grown to after the time period is called the accumulated value and is denoted by A(t) – amount function.
t ≥0 is measured in years (for the moment)
Define Accumulation function a(t)=A(t)/A(0)
A(0)=principal
a(0)=1
A(t)=A(0)∙a(t)

5. Natural assumptions on a(t)
increasing
(piece-wise) continuous
(0,1)
a(t) t
(0,1)
a(t) t
(0,1) t
a(t)
Note: a(0)=1

6. Definition of Interest and Rate of Interest
Interest = Accumulated Value – Principal: Interest = A(t) – A(0)
Effective rate of interest i (per year):
Effective rate of interest in nth year in:

7. Example (p. 5) a(t)=t2+t+1 Verify that a(0)=1
Show that a(t) is increasing for all t ≥ 0
Is a(t) continuous?
Find the effective rate of interest i for a(t)
Find in

8. ( ≡ Two Types of Accumulation Functions)
Two Types of Interest
( ≡ Two Types of Accumulation Functions)
Simple interest:
only principal earns interest
beneficial for short term (1 year)
easy to describe
Compound interest:
interest earns interest
beneficial for long term
the most important type of accumulation function

9. 1.2 SIMPLE INTEREST a(t)=1+it, t ≥0
(0,1) t
a(t) =1+it 1
1+i
Amount function: A(t)=A(0) ∙a(t)=A(0)(1+it)
Effective rate is i
Effective rate in nth year:

10. a(t)=1+it Example (p. 5) A(0)=1000 i=0.15
Jack borrows 1000 from the bank on January 1, 1996 at a rate of 15% simple interest per year. How much does he owe on January 17, 1996?
Solution
A(0)=1000
i=0.15
A(t)=A(0)(1+it)=1000(1+0.15t)
t=?

11. How to calculate t in practice?
Exact simple interest number of days
Ordinary simple interest (Banker’s Rule) number of days
t = t=
Number of days: count the last day but not the first

12. A(t)=1000(1+0.15t) Exact simple interest
Number of days (from Jan 1 to Jan 17) = 16
Exact simple interest
t=16/365
A(t)=1000( ∙ 16/365) =
Ordinary simple interest (Banker’s Rule)
t=16/360
A(t)=1000( ∙ 16/360) =

13. 1.3 COMPOUND INTEREST Interest earns interest
After one year: a(1) = 1+i
After two years: a(2) = 1+i+i(1+i) = (1+i)(1+i)=(1+i)2
Similarly after n years: a(n) = (1+i)n

14. a(t)=(1+i)t COMPOUND INTEREST Accumulation Function
Amount function: A(t)=A(0) ∙a(t)=A(0) (1+i)t
Effective rate is i
Moreover effective rate in nth year is i (effective rate is constant):
a(t)=(1+i)t
(0,1) t
a(t)=(1+i)t 1
1+i
1+it

15. How to evaluate a(t)?
If t is not an integer, first find the value for the integral values immediately before and after
Use linear interpolation
Thus, compound interest is used for integral values of t and simple interest is used between integral values 1 t
a(t)=(1+i)t
1+i 2
(1+i)2

16. Example (p. 8) a(t)=(1+i)t A(t)=A(0)(1+i)t A(t)=1000(1+0.15)t
Jack borrows 1000 at 15% compound interest.
How much does he owe after 2 years?
How much does he owe after 57 days, assuming compound interest between integral durations?
How much does he owe after 1 year and 57 days, under the same assumptions as in (b)?
How much does he owe after 1 year and 57 days, assuming linear interpolation between integral durations
In how many years will his principal have accumulated to 2000?
a(t)=(1+i)t
A(t)=A(0)(1+i)t
A(0)=1000, i=0.15
A(t)=1000(1+0.15)t

17. 1.4 PRESENT VALUE AND DISCOUNT
Definition
The amount of money that will accumulate to the principal over t years is called the present value t years in the past.
PRESENT VALUE
PRINCIPAL
ACCUMULATED VALUE

18. Calculation of present value
t=1, principal = 1
Let v denote the present value
v (1+i)=1
v=1/(1+i)

19. In general: v=1/(1+i) t is arbitrary a(t)=(1+i)t
[the present value of 1 (t years in the past)]∙ (1+i)t = 1
the present value of 1 (t years in the past) = 1/ (1+i)t = vt

20. a(t)=(1+i)t gives the value of one unit (at time 0) at any time t, past or future
(0,1) t
a(t)=(1+i)t

21. If principal is not equal to 1…
present value = A(0) (1+i)t
t < 0
t = 0
t > 0
PRESENT VALUE A(0) (1+i)t
PRINCIPAL A (0)
ACCUMULATED VALUE A(0) (1+i)t

22. Example (p. 11) Solution a(t)=(1+i)t
The Kelly family buys a new house for 93,500 on May 1, How much was this house worth on May 1, 1992 if real estate prices have risen at a compound rate for 8 % per year during that period?
Solution
a(t)=(1+i)t
Find present value of A(0) = 93, = 4 years in the past
t = - 4, i = 0.08
Present value = A(0) (1+i)t = 93,500 (1+0.8) = 68,725.29

23. If simple interest is assumed…
a (t) = 1 + it
Let x denote the present value of one unit t years in the past
x ∙a (t) = x (1 + it) =1
x = 1 / (1 + it)
NOTE:
In the last formula, t is positive
t > 0

24. Thus, unlikely to the case of compound interest, we cannot use the same formula for present value and accumulated value in the case of simple interest
a(t) =1+it
a(t) =1+it 1 t 1 t
1 / (1 - it)
1 / (1 + it)

25. Discount Look at 112 as a basic amount We invest 100
Alternatively:
Look at 112 as a basic amount
Imagine that 12 were deducted from 112 at the beginning of the year
Then 12 is amount of discount
We invest 100
After one year it accumulates to 112
The interest 12 was added at the end of the term

26. Rate of Discount d = i = Definition Effective rate of discount d
accumulated value after 1 year – principal accumulated value after 1 year
A(1) – A(0) A(1)
d = =
A(0) ∙a(1)– A(0) A(0) ∙a(1)
a(1) – 1 a(1) = =
Recall:
accumulated value after 1 year – principal principal
i =
a(1) – 1 a(0) =

27. In nth year…

28. Identities relating d to i and v
Note: d < i

29. Present and accumulated values in terms of d:
Present value = principal * (1-d)t
Accumulated value = principal * [1/(1-d)t]
If we consider positive and negative values of t then:
a(t) = (1 - d)-t

30. Examples (p. 13)
1000 is to be accumulated by January 1, 1995 at a compound rate of discount of 9% per year.
Find the present value on January 1, 1992
Find the value of i corresponding to d
Jane deposits 1000 in a bank account on August 1, If the rate of compound interest is 7% per year, find the value of this deposit on August 1, 1994.

31. 1.5 NOMINAL RATE OF INTEREST
Example (p. 13)
A man borrows 1000 at an effective rate of interest of 2% per month. How much does he owe after 3 years?
Note: t is the number of effective interest periods in any particular problem

32. More examples… (p. 14)
You want to take out a mortgage on a house and discover that a rate of interest is 12% per year. However, you find out that this rate is “convertible semi-annually”. Is 12% the effective rate of interest per year?
Credit card charges 18% per year convertible monthly. Is 18% the effective rate of interest per year?
Note: in both examples the given rates of interest (12% and 18%) were nominal rates of interest

33. Definition Suppose we have interest convertible m times per year
The nominal rate of interest i(m) is defined so that i(m) / m is an effective rate of interest in 1/m part of a year

34. Note: If i is the effective rate of interest per year, it follows that
In other words, i is the effective rate of interest convertible annually which is equivalent to the effective rate of interest i(m) /m convertible mthly.
Equivalently:

35. Examples (p. 15)
Find the accumulated value of 1000 after three years at a rate of interest of 24 % per year convertible monthly
If i(6)=15% find the equivalent nominal rate of interest convertible semi-annually

36. Nominal rate of discount
The nominal rate of discount d(m) is defined so that d(m) / m is an effective rate of interest in 1/m part of a year
Formula:

37. Formula relating nominal rates of interest and discount

38. Example
Find the nominal rate of discount convertible semiannualy which is equivalent to a nominal rate of interest of 12% convertible monthly

39. 1.6 FORCE OF INTEREST
What happens if the number m of periods is very large?
One can consider mathematical model of interest which is convertible continuously
Then the force of interest is the nominal rate of interest, convertible continuously

40. Definition Nominal rate of interest equivalent to i:
Let m approach infinity:
We define the force of interest δ equal to this limit:

41. Formula Force of interest δ = ln (1+i) Therefore eδ = 1+i
and a (t) = (1+i)t =eδt
Practical use of δ: the previous formula gives good approximation to a(t) when m is very large

42. Example
A loan of 3000 is taken out on June 23, If the force of interest is 14%, find each of the following:
The value of the loan on June 23, 2002
The value of i
The value of i(12)

43. Remark
The last formula shows that it is reasonable to define force of interest for arbitrary accumulation function a(t)

44. Definition Note: in general case, force of interest depends on t
The force of interest corresponding to a(t):
Note:
in general case, force of interest depends on t
it does not depend on t ↔ a(t)= (1+i)t !

45. Example (p. 19)
Find in δt the case of simple interest
Solution

46. How to find a(t) if we are given by δt ?
We have:
Consider differential equation in which a = a(t) is unknown function:
Since a(0) = 1 its solution is given by

47. Applications
Prove that if δt = δ is a constant then a(t) = (1+i)t for some i
Prove that for any amount function A(t) we have:
Note: δt dt represents the effective rate of interest over the infinitesimal “period of time” dt . Hence A(t)δt dt is the amount of interest earned in this period and the integral is the total amount

48. Remarks Do we need to define the force of discount?
It turns out that the force of discount coincides with the force of interest! (Exercise: PROVE IT)
Moreover, we have the following inequalities:
and formulas: