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Concrete mathematics : a foundation for computer science

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hence our tough question has a surprisingly simple answer:<br />

[znl G(z)’ = (“I+‘) $1<br />

7.5 CONVOLUTIONS 349<br />

<strong>for</strong> all integers 1 > 0.<br />

Readers who haven’t <strong>for</strong>gotten Chapter 5 might well be experiencing dkjjh<br />

vu: “That <strong>for</strong>mula looks familiar; haven’t we seen it be<strong>for</strong>e?” Yes, indeed;<br />

equation (5.60) says that<br />

[z”]B,(z)’ = -Jr &.<br />

( )<br />

There<strong>for</strong>e the generating function G(z) in (7.68) must actually be the generalized<br />

binomial series ‘B,(z). Sure enough, equation (5.59) says<br />

cBm(z)‘-m - Tim(z)-” = 2)<br />

which is the same as<br />

T3B(z)-l = zB,(z)"<br />

Let’s switch to the notation of Chapter 5, now that we know we’re dealing<br />

with generalized binomials. Chapter 5 stated a bunch of identities without<br />

proof. We have now closed part of the gap by proving that the power series<br />

IBt (z) defined by<br />

TQ(z) = x y &<br />

n ( 1<br />

has the remarkable property that<br />

%(z)’ = x (yr)$&,<br />

n<br />

whenever t and T ;Ire positive integers.<br />

Can we extend these results to arbitrary values oft and I-? Yes; because<br />

the coefficients (t:T’) & are polynomials in t and T. The general rth power<br />

defined by<br />

‘B,(z)’ = erln’Bt(z) - -9 rln93t(z))n<br />

ll20<br />

n!<br />

= t $ (- 2 (I-y)nl)‘,<br />

ll>O llI>l<br />

has coefficients that are polynomials in t and r; and those polynomials are<br />

equal to (tnn+‘) &; <strong>for</strong> infinitely many values oft and r. So the two sequences<br />

of polynomials must be identically equal.

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