Recurrence Relations Section 6 1 Definition A recurrence

Recurrence Relations Section 6. 1

Definition • A recurrence relation for the sequence {an} is an equation that expresses an in terms of one or more of the previous terms of the sequence, namely, a 0, a 1, …, an-1, for all integers n with n n 0, where n 0 is a nonnegative integer. • A sequence is called a solution of a recurrence relation if its terms satisfy the recurrence relation. CSE 2813 Discrete Structures

Recurrence Relations vs. Recursive Definitions • So what is the difference? • Recursive definitions can be used to solve counting problems. When they are used in this way, the rule for finding terms from those that precede them is called a recurrence relation. CSE 2813 Discrete Structures

Example • Let {an} be a sequence that satisfies the recurrence relation an = an-1 an-2 for n = 2, 3, 4, … Suppose that a 0 = 3 and a 1 = 5. • What are a 2 and a 3? CSE 2813 Discrete Structures

Example • Consider the recurrence relation: an = 2 an-1 an-2 for n = 2, 3, 4, … • Show whether each of the following is a solution of this recurrence relation? an = 3 n an = 2 n an = 5 CSE 2813 Discrete Structures

Modeling with Recurrence Relations • A person deposits $10, 000 in a savings account at a bank yielding 11% per year with interest compounded annually. • How much will be in the account after 30 years? CSE 2813 Discrete Structures

Rabbits and the Fibonacci Sequence • A young pair of rabbits (one of each sex) is placed on an island. – A pair does not breed until they are 2 months old. – After they are 2 months old, each pair produces another pair each month. • Find the number of pairs of rabbits on the island after n months, assuming that no rabbits ever die. CSE 2813 Discrete Structures

The Tower of Hanoi • Find a recurrence relation to find the number of moves needed to solve the Tower of Hanoi problem with n disks. Tower of Hanoi CSE 2813 Discrete Structures

More Example • Find a recurrence relation for the number of bit strings of length n that do not contain two consecutive 0 s. • Find a recurrence relation for the number of bit strings of length n that contain two consecutive 0 s. CSE 2813 Discrete Structures

Exercises • 1, 5, 9, 10, 11, 13, 24, 25, 27 CSE 2813 Discrete Structures