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Problem Set 1 Solution

1.    Prove that 4n + 15n – 1 is divisible by 9 for all n ≥ 1, using simple induction.

(3 marks)

    2. Consider binary strings that start with a 1. By interpreting the 1's as specifying powers of 2, these strings are called binary representations of positive integers. For example:
10=21=2

1011=23+21+20=11

110001 = 25 + 24 + 20 = 49

a)    Prove that every natural number n ≥ 1 has a binary representation.    (4 marks)

b)  Prove that the binary representation is unique.    (4 marks)

3.    Consider the Fibonacci-esque function g:


1    if n=0


3    if n=1

g(n – 2) + g(n – 1)    if n > 1

Use complete induction to prove that if n is a natural number greater than 1, then 2n/2 ≤ g(n) ≤ 2n.

(7 marks)

4.    Given the function L(n) below:


int L(n) :

if n < 7 : return 0

else : return 1 + L(n/7)

find a recurrence that expresses the time complexity, T(n) of L(n), in terms of n. Find a closed form for your recurrence and prove it is correct. Your expression should assign a constant to operations that do not depend on n.

(6 marks)

    5. Let set F be recursively defined as follows:

        a. 7 ∈ F

        b. If u, v ∈ F, then u + v ∈ F

        c. Nothing else is in F

Use structural induction to prove that ∀w ∈ F, w mod 7 = 0, i.e., all elements in F are divisible by 7.

(6 marks)


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