1 CSE 20 Lecture 11 Function, Recursion & Analysis (Ch. 6 Shaum’s ) May 3, 2011

Motivation Complexity of the execution 2 Input x Output y y= Function (x) Recursion

OUTLINE DEFINITION FUNTION RECURSION: CASES ANALYSIS 3

4 I. Definition A function f: A → B maps elements in domain A to codomain B such that for each a ϵ A, f(a) is exact one element in B. f: A → B A: Domain B: Codomain f(A): range or image of function f(A) ⊂ B

5 Examples (2) f(x) (1) f(x)=x 2, x ∈ ℝ (3) f(x) NOT a function

6 (5) NOT a function Examples (4) Mapping from x to y When domain A is an integer set Z, we may denote f(x) as f x, ie. f 0, f 1, f 2.

7 Case of Recursion Fibonacci Sequence Index: Sequence:

8 Another Case of Recursion: Ackermann Function Ackermann Function: A(m,n), m, n ∈ N 1) m=0: A(0,n)=n+1 2) m≠0, n=0: A(m,0)=A(m-1,1) 3) m≠0, n≠0: A(m,n)=A(m-1, A(m,n-1)) Example: A(1,1)= A(0, A(1,0)) =A(0,2) A(2,0) =A(1,1)

9 Ackermann Function (0,0) (0,1) (0,2) (0,3) (0,4) (0,5) (0,6) (0,7) A(0,n)=n+1 (1,0) (1,1) (1,2) (1,3) (1,4) (1,5) (1,6) (1,7) A(1,n)=n+2 (2,0) (2,1) (2,2) (2,3) (2,4)(2,5) A(2,n)=2n+3 (3,0) (3,1) (3,2) A(3,n)= (4,0)(4,1)

10 Ackermann Function - Comparison Ackermann Function: grows faster than an exponential function Googol: Visible universe ≈ atoms

11 Fibonacci Sequence Fibonacci sequence: Start with a pair of rabbits. Every month each pair bears a pair, which becomes productive from the second month. Calculate the number of new pairs in month i, i≥0.

12 Analysis of Fibonacci Sequence Algorithm array n(i) (new born), a(i) (adult) Init n(0)=0, a(0)=1 For i=1, i=i+1 n(i)=a(i-1) a(i)=a(i-1)+n(i-1) Index: … a(i): … n(i): …

13 Fibonacci sequence: Golden Ratio Derivation: Let We have:

14 Fibonacci Sequence and the golden ratio