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Concrete mathematics : a foundation for computer science

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516 ANSWERS TO EXERCISES<br />

5.44 Cancellation of factorials shows that<br />

(;)(;)(“;“) == (m+;:;-k)(j;k)(y;;).<br />

so the second sum is l/( “:“I I times the first. We can show that the first sum<br />

is (“ib) (“-~P~~b), whenever n 3 b, even if m < a: Let a and b be fixed<br />

and call the first sum S( m, ‘1). Identity (5.32) covers the case n = b, and<br />

we have S(m,n) = S(m,n - 1) + S(m- 1,n) + (-l)m+n(mzn)(i)(“) since<br />

(m+;:;-k) = r+:;J;j--k) + (m-;‘;r;-k). The result follows by induction on<br />

m+ n, since (,) = 0 when n > b and the case m = 0 is trivial. By symmetry,<br />

the <strong>for</strong>mula (“ib) (m+l:im “) holds whenever m > a, even if n < b.<br />

5.45 According to (5.g), xc

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