# week5 - S Tanny Recurrence Relations Tower of Hanoi Let Tn...

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S. Tanny MAT 344 Spring 1999 72 Recurrence Relations Tower of Hanoi Let T n be the minimum number of moves required. T 0 = 0, T 1 = 1 7 Initial Conditions * T n = 2 T n - 1 + 1 n $2 T n is a sequence (fn. on integers). Solve for T n ? * is a recurrence , difference equation (linear, non-homogeneous, constant coefficient) Set U 0 = T 0 + 1 , U n = T n + 1 n$ 1 Then U n = T n + 1 = 2T n - 1 + 1 + 1 = 2(T n - 1 + 1) so U n = 2U n - 1 = 2 2 U n - 2 = = 2 n - 1 U 1 = 2 n ± T n = 2 n - 1 Suppose a n + 1 = 2a n + n , a 0 = 1 Let U n = a n + n , U 0 = 1 Then U n + 1 = a n + 1 + (n + 1) = 2a n + n + (n + 1) = 2(a n + n) + 1 = 2 U n + 1 Thus, U n is like the T n in the preceding example, except U 0 = 1 while T 0 = 0. In fact, since T 1 = 1, the {U n } is just {T n } “advanced one step”, i.e. U n = T n + 1 = 2 n + 1 - 1 ± a n = 2 n + 1 - 1 - n = 2 n + 1 - (n + 1) Notice how the solution of one recurrence often can be reduced to the solution of a simpler one.

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S. Tanny MAT 344 Spring 1999 73 Suppose the recursion were L n = L n - 1 + n , L 0 = 1 Then we can “expand out ” as follows: L n = L n - 2 + (n - 1) + n = L n - 3 + (n - 2) + (n - 1) + n = = L 0 + 1 + 2 + + n = 1 + n(n % 1) 2 This describes the number of regions formed by n intersecting lines in the plane, no 2 parallel and no 3 intersect in a point (PIZZA CUTTING PROBLEM). General Problem (*) F(Y n + k ,Y n + k - 1 , , Y n ) = 0 Difference equation of order k(DFE) Assume F linear , constant coefficients (**) Y n + k + a 1 Y n + k - 1 + + a k Y n - n (n) = 0 If n (n) = 0, Homogeneous; otherwise non-Homo. Note the strong analogy with D.E.! Suppose that Y n = S 1 (n) is a “solution”. Then S 1 (n + k) + a 1 S 1 (n + k - 1) + + a k S 1 (n) - n (n)=0 If S 2 (n) is any other solution, then [S 1 (n + k) - S 2 (n + k)] + a 1 [S 1 ( n + k - 1) - S 2 (n+ k - 1)] + ± + a k [S 1 (n) - S 2 (n)] = 0 It follows that S 1 (n) - S 2 (n) is a solution of the Homogeneous equation related to (**) (obtained by (ignoring n (n)). What is the “General Solution” of a DFE? It’s a family of functions, usually characterized by a parameter(s) which can take on different values.
S. Tanny MAT 344 Spring 1999 74 From the above, the general solution for (**) is just the general solution to the related HOMO equation plus any particular solution to the NON-HOMO equation (**), i.e. S NH (n) = S H (n) + S p (n) where S p (n) is any solution of (**), S H (n) is general solution of related HOMO and S NH (n) is general solution of (**) Solving HOMO (i) First Order DFE Suppose Y n + 1 + a 1 Y n = 0 (k = 1) Then Y n + 1 = -a 1 Y n = (-1) 2 a 1 2 Y n - 1 = = (-1) n a 1 n Y 1 = (-a 1 ) n + 1 Y 0 where Y

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week5 - S Tanny Recurrence Relations Tower of Hanoi Let Tn...

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