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Concrete mathematics : a foundation for computer science

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564 ANSWERS TO EXERCISES<br />

8.56 If m is even, the frisbees always stay an odd distance apart and the<br />

game lasts <strong>for</strong>ever. If m = i:l. + 1, the relevant generating functions are<br />

(The coefficient [z”] Ak is the probability that the distance between frisbees<br />

is 2k after n throws.) Taking a clue from the similar equations in exercise 49,<br />

we set z = 1 /cos’ 8 and Al := X sin28, where X is to be determined. It follows<br />

by induction (not using the equation <strong>for</strong> Al) that Ak = X sin2kO. There<strong>for</strong>e<br />

we want to choose X such that<br />

(<br />

3<br />

1-p X sin;!10 = 1 +& X sin(21- 218.<br />

4cos28 ><br />

It turns out that X = 2 cos’ O/sin 8 cos(21+ 1 )O, hence<br />

cos e<br />

G, = - cos me ’<br />

The denominator vanishes when 8 is an odd multiple of n/(2m); thus 1 -qkz is<br />

a root of the denominator <strong>for</strong> 1 6 k 6 1, and the stated product representation<br />

must hold. To find the mean and variance we can write Trigonometry wins<br />

again. is there a<br />

G, = (1 - $2 + L.04 - . . )/(I - $2@ + &m4@4 - . . . ) connection with<br />

pitching pennies<br />

= 1 + i(m2 - 1)02 + &(5m4 -6m2+ 1)04 +...<br />

= 1 +~(m2-l)(tanB)2+~(5m4-14m2+9)(tan8)4+~~~<br />

= 1 + G:,(l)(tan8)2 + iGK(1)(tan8)4 +... ,<br />

because tan28 = Z- 1 and tan8 =O+ i03 +.... So we have Mean =<br />

i(m2-1) andVar(G,) = im2(m2-1). (Note that thisimplies theidentities<br />

m2 - 1<br />

~ =<br />

2<br />

(m-1 l/2<br />

& i = 'mjf'2(l/sin "",r,,')")';<br />

lm Ii/2<br />

m2(m2 - 1)<br />

- cot (2k- 1)~ (2k- 1)n 2<br />

6 = u 2m I sin<br />

2m > ’<br />

k=l<br />

The third cumulant of this distribution is &m2(m2 - l)(4m2 - 1); but the<br />

pattern of nice cumulant factorizations stops there. There’s a much simpler<br />

k=l<br />

along the angles of<br />

the m-gon?

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