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Concrete mathematics : a foundation for computer science

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408 DISCRETE PROBABILITY<br />

Complicated, yes; but everything simplifies greatly when we set z = 1. For<br />

example, we have<br />

t<br />

n30<br />

Wk<br />

B;(l)y zz m,+m ‘)y<br />

k>2<br />

/<br />

2(k - 2)!<br />

and it follows that<br />

= me{” ‘)w<br />

wkf2<br />

- -<br />

lx<br />

k>O 2k!<br />

,,z,im llw 1Y<br />

E--e =<br />

2<br />

( mw)n+2 n(n-l)m”wn<br />

2mn! = IL 2mn! ’<br />

x<br />

TX30 II30<br />

The expression <strong>for</strong> EA in (8.!3g) now gives EA = 1 + (n- 1)/2m, in agreement<br />

with (8.96).<br />

The <strong>for</strong>mula <strong>for</strong> Bz (1) involves the similar sum<br />

5 (;) (G>-,) g = f & (k+ ‘)k(k;;)(-“<br />

/ ,<br />

hence we find that<br />

= mewm(+mw4 + w”) ;<br />

B;(l) = (;:)((1) -l>s*<br />

(8.101)<br />

Now we can put all the pieces together and evaluate the desired variance VA.<br />

Massive cancellation occurs, and the result is surprisingly simple:<br />

B”(1) + B;(l) - B;(1)2<br />

VA+L n2<br />

=--<br />

(m- l)i:n- 1)<br />

=- 2mln<br />

(n+l)(n-2) +-m-n(n-1)<br />

4 2 4<br />

(8.102)

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