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Concrete mathematics : a foundation for computer science

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504 ANSWERS TO EXERCISES<br />

4.40 Let f(n) = n,,,,,,,,, k = n!/pl"/pJ Ln/p]! and g(n) = n!/pEP(“!l.<br />

Then<br />

s(n) = f(n)f( ln/PJ) f( ln/p’J) . . . = f(n) g( b/d) .<br />

Also f(n) = ao!(p - l)!Ln/pl = ao!(-l)L"/PJ (mod p), and e,(n!) = Ln/pJ +<br />

cp (Ln/pJ !) . These recurrences make it easy to prove the result by induction.<br />

(Several other solutions are possible.)<br />

4.41 (a) If n2 = -1 (mod p) then (n2)(pP’i/2 = -1; but Fermat says it’s<br />

+l. (b) Let n = ((p - 1)/2)!; we have n = (--l)(P~‘i’2 n,sk n.1 or rn2 < n2 do<br />

if rnz < n2 then (output(L); (nl,nz) := (nl,nz) - (ml,m,))<br />

else (output(R); (ml, m2) := (ml, ml) - (nl ,nz)) .<br />

The output is LLLRRRRR = & z .3158. Incidentally, an average of .334<br />

implies at least 287 at bats.

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