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0bb?-O=xz$Now it s Time for& $ Recurrence
Relations
.66f3 {Recurrence Relations$ A recurrence relation for the sequence {an} is an equation that expresses an is terms of one or more of the previous terms of the sequence, namely, a0, a1, & , an-1, for all integers n with n n0, where n0 is a nonnegative integer.
A sequence is called a solution of a recurrence relation if it terms satisfy the recurrence relation.
Q!I 3HQ |Recurrence Relations$ ZIn other words, a recurrence relation is like a recursively defined sequence, but without specifying any initial values (initial conditions).
Therefore, the same recurrence relation can have (and usually has) multiple solutions.
If both the initial conditions and the recurrence relation are specified, then the sequence is uniquely determined.[ZR:FX [ }Recurrence Relations$ LExample: Consider the recurrence relation an = 2an-1 an-2 for n = 2, 3, 4, &
Is the sequence {an} with an=3n a solution of this recurrence relation?
For n 2 we see that 2an-1 an-2 = 2(3(n 1)) 3(n 2) = 3n = an.
Therefore, {an} with an=3n is a solution of the recurrence relation.d'%E"
f3f3f3f3.f3' ~Recurrence Relations$ Is the sequence {an} with an=5 a solution of the same recurrence relation?
For n 2 we see that 2an-1 an-2 = 25 - 5 = 5 = an.
Therefore, {an} with an=5 is also a solution of the recurrence relation.H
f3f3f3f32f3 "Modeling with Recurrence Relations##$" Example:
Someone deposits $10,000 in a savings account at a bank yielding 5% per year with interest compounded annually. How much money will be in the account after 30 years?
Solution:
Let Pn denote the amount in the account after n years.
How can we determine Pn on the basis of Pn-1?
GZ E !"Modeling with Recurrence Relations##$" vWe can derive the following recurrence relation:
Pn = Pn-1 + 0.05Pn-1 = 1.05Pn-1.
The initial condition is P0 = 10,000.
Then we have:
P1 = 1.05P0
P2 = 1.05P1 = (1.05)2P0
P3 = 1.05P2 = (1.05)3P0
&
Pn = 1.05Pn-1 = (1.05)nP0
We now have a formula to calculate Pn for any natural number n and can avoid the iteration.\<Z 7t0 ; 7 ""Modeling with Recurrence Relations##$" DLet us use this formula to find P30 under the
initial condition P0 = 10,000:
P30 = (1.05)3010,000 = 43,219.42
After 30 years, the account contains $43,219.42.!
1 #"Modeling with Recurrence Relations##$" Another example:
Let an denote the number of bit strings of length n that do not have two consecutive 0s ( valid strings ). Find a recurrence relation and give initial conditions for the sequence {an}.
Solution:
Idea: The number of valid strings equals the number of valid strings ending with a 0 plus the number of valid strings ending with a 1.]
] $"Modeling with Recurrence Relations##$" FLet us assume that n 3, so that the string contains at least 3 bits.
Let us further assume that we know the number an-1 of valid strings of length (n 1).
Then how many valid strings of length n are there, if the string ends with a 1?
There are an-1 such strings, namely the set of valid strings of length (n 1) with a 1 appended to them.
Note: Whenever we append a 1 to a valid string, that string remains valid.hv\3F %"Modeling with Recurrence Relations##$" Now we need to know: How many valid strings of length n are there, if the string ends with a 0?
Valid strings of length n ending with a 0 must have a 1 as their (n 1)st bit (otherwise they would end with 00 and would not be valid).
And what is the number of valid strings of length (n 1) that end with a 1?
We already know that there are an-1 strings of length n that end with a 1.
Therefore, there are an-2 strings of length (n 1) that end with a 1.],$>.& &"Modeling with Recurrence Relations##$" So there are an-2 valid strings of length n that end with a 0 (all valid strings of length (n 2) with 10 appended to them).
As we said before, the number of valid strings is the number of valid strings ending with a 0 plus the number of valid strings ending with a 1.
That gives us the following recurrence relation:
an = an-1 + an-2RmR '"Modeling with Recurrence Relations##$" 2What are the initial conditions?
a1 = 2 (0 and 1)
a2 = 3 (01, 10, and 11)
a3 = a2 + a1 = 3 + 2 = 5
a4 = a3 + a2 = 5 + 3 = 8
a5 = a4 + a3 = 8 + 5 = 13
&
This sequence satisfies the same recurrence relation as the Fibonacci sequence.
Since a1 = f3 and a2 = f4, we have an = fn+2.
J
= &
9 (Solving Recurrence Relations$ DIn general, we would prefer to have an explicit formula to compute the value of an rather than conducting n iterations.
For one class of recurrence relations, we can obtain such formulas in a systematic way.
Those are the recurrence relations that express the terms of a sequence as linear combinations of previous terms.E'&XKE )Solving Recurrence Relations$ Definition: A linear homogeneous recurrence relation of degree k with constant coefficients is a recurrence relation of the form:
an = c1an-1 + c2an-2 + & + ckan-k,
Where c1, c2, & , ck are real numbers, and ck 0.
A sequence satisfying such a recurrence relation is uniquely determined by the recurrence relation and the k initial conditions
a0 = C0, a1 = C1, a2 = C2, & , ak-1 = Ck-1.0Zx @ *Solving Recurrence Relations$ 1Examples:
The recurrence relation Pn = (1.05)Pn-1
is a linear homogeneous recurrence relation of degree one.
The recurrence relation fn = fn-1 + fn-2
is a linear homogeneous recurrence relation of degree two.
The recurrence relation an = an-5
is a linear homogeneous recurrence relation of degree five.Z'0Z
/
/
/@" +Solving Recurrence Relations$ tBasically, when solving such recurrence relations, we try to find solutions of the form an = rn, where r is a constant.
an = rn is a solution of the recurrence relationan = c1an-1 + c2an-2 + & + ckan-k if and only if
rn = c1rn-1 + c2rn-2 + & + ckrn-k.
Divide this equation by rn-k and subtract the right-hand side from the left:
rk - c1rk-1 - c2rk-2 - & - ck-1r - ck = 0
This is called the characteristic equation of the recurrence relation.X+2\ D 3 o ,Solving Recurrence Relations$ The solutions of this equation are called the characteristic roots of the recurrence relation.
Let us consider linear homogeneous recurrence relations of degree two.
Theorem: Let c1 and c2 be real numbers. Suppose that r2 c1r c2 = 0 has two distinct roots r1 and r2.
Then the sequence {an} is a solution of the recurrence relation an = c1an-1 + c2an-2 if and only if an = a1r1n + a2r2n for n = 0, 1, 2, & , where a1 and a2 are constants.
See pp. 321 and 322 for the proof.Z. ;
, #f3 -Solving Recurrence Relations$ Example: What is the solution of the recurrence relation an = an-1 + 2an-2 with a0 = 2 and a1 = 7 ?
Solution: The characteristic equation of the recurrence relation is r2 r 2 = 0.
Its roots are r = 2 and r = -1.
Hence, the sequence {an} is a solution to the recurrence relation if and only if:
an = a12n + a2(-1)n for some constants a1 and a2._2
<D<_ .Solving Recurrence Relations$ Given the equation an = a12n + a2(-1)n and the initial conditions a0 = 2 and a1 = 7, it follows that
a0 = 2 = a1 + a2
a1 = 7 = a12 + a2 (-1)
Solving these two equations gives usa1 = 3 and a2 = -1.
Therefore, the solution to the recurrence relation and initial conditions is the sequence {an} with
an = 32n (-1)n. 4B
&
\B /Solving Recurrence Relations$ an = rn is a solution of the linear homogeneous recurrence relationan = c1an-1 + c2an-2 + & + ckan-k
if and only if
rn = c1rn-1 + c2rn-2 + & + ckrn-k.
Divide this equation by rn-k and subtract the right-hand side from the left:
rk - c1rk-1 - c2rk-2 - & - ck-1r - ck = 0
This is called the characteristic equation of the recurrence relation.W>2 X 3 o 0Solving Recurrence Relations$ The solutions of this equation are called the characteristic roots of the recurrence relation.
Let us consider linear homogeneous recurrence relations of degree two.
Theorem: Let c1 and c2 be real numbers. Suppose that r2 c1r c2 = 0 has two distinct roots r1 and r2.
Then the sequence {an} is a solution of the recurrence relation an = c1an-1 + c2an-2 if and only if an = a1r1n + a2r2n for n = 0, 1, 2, & , where a1 and a2 are constants.
See pp. 321 and 322 for the proof.Z. ;
, #f3 1Solving Recurrence Relations$ Example: Give an explicit formula for the Fibonacci numbers.
Solution: The Fibonacci numbers satisfy the recurrence relation fn = fn-1 + fn-2 with initial conditions f0 = 0 and f1 = 1.
The characteristic equation is r2 r 1 = 0.
Its roots are5 8
&@)
2Solving Recurrence Relations$ .Therefore, the Fibonacci numbers are given by "/- &
3Solving Recurrence Relations$ HThe unique solution to this system of two equations and two variables is"IH I 4Solving Recurrence Relations$ :But what happens if the characteristic equation has only one root?
How can we then match our equation with the initial conditions a0 and a1 ?
Theorem: Let c1 and c2 be real numbers with c2 0. Suppose that r2 c1r c2 = 0 has only one root r0. A sequence {an} is a solution of the recurrence relation an = c1an-1 + c2an-2 if and only if an = a1r0n + a2nr0n, for n = 0, 1, 2, & , where a1 and a2 are constants., 5Solving Recurrence Relations$ jExample: What is the solution of the recurrence relation an = 6an-1 9an-2 with a0 = 1 and a1 = 6?
Solution: The only root of r2 6r + 9 = 0 is r0 = 3.Hence, the solution to the recurrence relation is
an = a13n + a2n3n for some constants a1 and a2.
To match the initial condition, we need
a0 = 1 = a1a1 = 6 = a13 + a23
Solving these equations yields a1 = 1 and a2 = 1.
Consequently, the overall solution is given by
an = 3n + n3n.2
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