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Concrete mathematics : a foundation for computer science

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Baseball fans: .367<br />

is also Ty Cobb’s<br />

lifetime batting<br />

average, the a//-time<br />

record. Can this be<br />

a coincidence?<br />

(Hey wait, you’re<br />

fudging. Cobb ‘s<br />

average was<br />

4191/11429 z<br />

.366699, while<br />

l/e z .367879.<br />

But maybe if<br />

Wade Boggs has<br />

a few really good<br />

seasons. . . )<br />

5.3 TRICKS OF THE TRADE 195<br />

How can we solve a recurrence like (5.4g)? Easy; it has the <strong>for</strong>m of (5.48),<br />

with g(n) = n! and f(k) = (-l)kki. Hence its solution is<br />

ni = (-l)“t<br />

k<br />

Well, this isn’t really a solution; it’s a sum that should be put into closed <strong>for</strong>m<br />

if possible. But it’s better than a recurrence. The sum can be simplified, since<br />

k! cancels with a hidden k! in (i), so let’s try that: We get<br />

?li = x n!il]“+k = n! x (-‘lk .<br />

Oik

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