Math 184 Homework 6 Fall 2022
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Math 184 Homework 6
Fall 2022
This homework is due on gradescope Friday November 11th at 11:59pm pacific time. Remember to justify your work even if the problem does not explicitly say so. Writing your solutions in LATEXis recommend though not required.
Question 1 (Binomial Theorem Identity, 20 points). Notice that (1−x2 )n = (1+x)n (1−x)n . By expanding both sides using the Binomial Theorem and comparing terms, derive a combinatorial identity.
Question 2 (Generating Functions for Polynomials, 20 points). Show that for m a non-negative integer that
nm xn = S(m,k)k! ( ) .
Question 3 (Finite Differences, 30 points). Given a sequence a(0),a(1),a(2), . . ., define the first finite difference to be another sequence ∆a(n) = a(n)−a(n−1) for n ≥ 1 . Define the kth finite difference ∆k a(n) to be the sequence obtained by applying the finite difference operation to the sequence a(i) a total of k times .
(a) Given the generating function A(x) = 又 a(n)xn give a formula for the generating function for its first finite difference B(x) = 又 ∆a(n)xn . [15 points]
(b) Show that for the sequence a(n) = nk for some integer k ≥ 0 that the (k+1)st finite difference ∆k+1a(n) is 0 for all sufficiently large n . Hint: You may want to use the result from Question 2 above . [15 points]
Question 4 (Generating Function Calculations, points). Derive formulas for the following generating func- tions:
(a) The sequence an is given by the recurrence a0 = a1 = a2 = 1 and an = an −2 + an −3 for all n ≥ 3 . Give a formula for the generating function A(x) = 又 an xn . [10 points]
(b) The sequence bn is given by the recurrence b0 = 1 and
bn = {
bn/2 −b(n− 1)/2
if n is even
if n is odd
for all n ≥ 0 . Show that the generating function B(x) = 又 bn xn satisfies B(x) = (1−x)B(x2 ) . Give a formula for B(x) as an infinite product. [10 points]
(c) The sequence cn satisfies the recurrence relation cn = 又 cm + n . Give a differential equation satisfied by the generating function C(x) = 又 cn xn . [10 points]
Question 5 (Extra credit, 1 point). Approximately how much time did you spend working on this homework?
2022-11-12