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Concrete mathematics : a foundation for computer science

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86 INTEGER FUNCTIONS<br />

3.5 FLOOR/CEILING SUMS<br />

Equation (3.26) demonstrates that it’s possible to get a closed <strong>for</strong>m<br />

<strong>for</strong> at least one kind of sum that involves 1 J. Are there others? Yes. The<br />

trick that usually works in such cases is to get rid of the floor or ceiling by<br />

introducing a new variable.<br />

For example, let’s see if it’s possible to do the sum<br />

in closed <strong>for</strong>m. One idea is to introduce the variable m = L&J; we can do<br />

this “mechanically” by proceeding as we did in the roulette problem:<br />

x l&J = t m[k

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