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Concrete mathematics : a foundation for computer science

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Do old exams<br />

ever die?<br />

5.2 BASIC PRACTICE 177<br />

The other sum A is the same, but with m replaced by m - 1. Hence we<br />

have a closed <strong>for</strong>m <strong>for</strong> the given sum S, which can be further simplified:<br />

S = mA-(m-n)B = m(mmn) -(m-n)(mrnn:,)<br />

= (m-Y+,) (mmn)’<br />

And this gives us a closed <strong>for</strong>m <strong>for</strong> the original sum:<br />

= m-n+1 m - n<br />

n ( m m<br />

n<br />

= m-n+1 ’<br />

Even the referee can’t simplify this.<br />

Again we use a small case to check the answer. When m = 4 and n = 2,<br />

we have<br />

T = ow(;) + lW@ + 24/(4,) = o+ g +; = 5)<br />

which agrees with our <strong>for</strong>mula 2/(4 - 2 + 1).<br />

Problem 3: From an old exam.<br />

Let’s do one more sum that involves a single binomial coefficient. This<br />

one, unlike the last, originated in the halls of academia; it was a problem on<br />

a take-home test. We want the value of Q~~OOOOO, when<br />

Qn = x (‘“k ‘)(-l)‘, integer n 3 0.<br />

k

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