# HW4 - CSE21 FA11 Homework#4 4.1 Prove by induction that n...

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CSE21 FA11 Homework #4 (10/17/11) 4.1. Prove by induction that n k =1 k 3 = ( n k =1 k ) 2 . (Hint: First get an explicit form form the second sum. Then use induction.) 4.2. ( Note: This problem is fairly tough! I’ll be impressed by anyone who can get this one!). You have the standard Tower of Hanoi situation (3 pegs and n discs of sizes 1 , 2 , 3 , . . . , n ). As usual, you can move one disc at a time from one peg to another one provided you don’t place a larger disc on top of a smaller one. In this variation, the pegs are labeled A , B and C . and you are restricted in how you can transfer the discs. Namely, you can only move discs from A to B , B to C , or C to A (so that you cannot move a disc from B to A , for example). Let D ( n ) denote the minimum number of moves needed to move a stack of n discs from A to B . For example, D (1) = 1 and D (2) = 5. Find a recurrence for D ( n ) and then solve the recurrence to get an explicit expression for D ( n ). What is the value of D (6)? 4.3. Find explicit solutions to the recurrences: (a) f ( n + 2) = 6 f ( n + 1) - 8 f ( n ) , n 0, with f (0) = 0 , f (1) = 1; (b) g ( n + 2) = 6 g ( n + 1) - 9 g ( n ) , n 0, with g (0) = 0 , g (1) = 1. 4.4 How many sequences of 0’s, 1’s and 2’s of length n are there which don’t have the string 00 in them? For example, for

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