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Concrete mathematics : a foundation for computer science

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272 SPECIAL NUMBERS<br />

trigonometric functions in terms of their hyperbolic cousins by using the rules<br />

sin z = -isinh iz , cos z = cash iz;<br />

the corresponding power series are<br />

sin2 = 2’ 1!-3!+5!--... 23 25 2’ 23 25<br />

, sinhz = T+“j-i.+5r+...;<br />

20 22 24<br />

cosz = o!-2!+4?--...)<br />

.ci .; zi<br />

coshz = ol+2r+T+... .<br />

. . .<br />

Hence cot z = cos z/sin z = i cash iz/ sinh iz = i coth iz, and we have<br />

Another remarkable <strong>for</strong>mula <strong>for</strong> zcot z was found by Euler (exercise 73):<br />

zcotz = l-2tTg.<br />

k>,krr -z2<br />

We can expand Euler’s <strong>for</strong>mula in powers of z2, obtaining<br />

.<br />

(6.86)<br />

(6.87)<br />

(6.88)<br />

Equating coefficients of zZn with those in our other <strong>for</strong>mula, (6.87), gives us<br />

an almost miraculous closed <strong>for</strong>m <strong>for</strong> infinitely many infinite sums:<br />

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