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Problem_Set_04

# Problem_Set_04 - ArsDigita University Month 2 Discrete...

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ArsDigita University Month 2: Discrete Mathematics - Professor Shai Simonson Problem Set 4 – Induction and Recurrence Equations 1. What’s wrong with the following proofs by induction? a. All binary strings are identical. The proof is by induction on the size of the string. For n= 0 all binary strings are empty and therefore identical. Let X = b n b n-1 …b 1 b 0 be an arbitrary binary string of length n+ 1. Let Y = b n b n-1 …b 1 and Z = b n-1 …b 1 b 0 . Since both Y and Z are strings of length less than n+ 1, by induction they are identical. Since the two strings overlap, X must also be identical to each of them. b. Any amount of change greater than or equal to twenty can be gotten with a combination of five cent and seven cent coins. The proof is by induction on the amount of change. For twenty cents use four five-cent coins. Let n > 20 be the amount of change. Assume that n = 7 x + 5 y for some non-negative integers x and y. For any n > 20, either x > 1, or y > 3. If x > 1, then since 3(5) – 2(7) = 1, n+ 1 = 5( y +3)+7( x –2). If y > 3, then since 3(7) – 4(5) = 1, n+ 1 = 7( x +3)+5( y –4). In either case, we showed that n+ 1 = 7 u + 5 v where u and v are non-negative integers. 2. Prove by induction that: a. The n th Fibonacci number equals (1/ 5)[(1/2 + 5/2) n – (1/2 – 5/2) n ], where F 0 = 0 and F 1 = 1.

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