# Page:Philosophical Transactions - Volume 145.djvu/192

173
mr. w.h.l. russell on the theory of definite integrals.

same factorials, so that we can deduce the value of many definite integrals from one series.

I shall now give an example of the summation of a factorial series of a somewhat different nature.

Consider the series—

$1+{\frac {x}{a^{2}+2^{2}}}+{\frac {x^{2}}{(a^{2}+2^{2})(a^{2}+4^{2})}}+{\frac {x^{3}}{(a^{2}+2^{2})(a^{2}+4^{2})\ldots (a^{2}+2^{2}n^{2})}}+\mathrm {\&c.,}$ we know that $\int _{-{\frac {\pi }{2}}}^{\frac {\pi }{2}}\varepsilon ^{a\theta }(\cos \theta )^{n}={\frac {1.2.4\ldots 2n}{(a^{2}+2^{2})(a^{2}+4^{2})\ldots (a^{2}+2^{2}n^{2})}}\cdot {\frac {\varepsilon ^{\frac {a\pi }{2}}-\varepsilon ^{-{\frac {a\pi }{2}}}}{a}}.$ Hence by substitution the above series becomes

{\begin{aligned}&{\frac {a}{\varepsilon ^{\frac {a\pi }{2}}-\varepsilon ^{-{\frac {a\pi }{2}}}}}\int _{-{\frac {\pi }{2}}}^{\frac {\pi }{2}}d\theta \varepsilon ^{a\theta }\left\{1+{\frac {x\cos ^{2}\theta }{1.2}}+{\frac {x^{2}\cos ^{4}\theta }{1.2.3.4}}+\mathrm {\&c.} \right\}\\&={\frac {a}{2\left(\varepsilon ^{\frac {a\pi }{2}}-\varepsilon ^{-{\frac {a\pi }{2}}}\right)}}\int _{-{\frac {\pi }{2}}}^{\frac {\pi }{2}}d\theta \varepsilon ^{a\theta }\{\varepsilon ^{{\sqrt {x}}\cos \theta }+\varepsilon ^{-{\sqrt {x}}\cos \theta }\}.\end{aligned}} There are other series of an analogous nature which may be summed in a similar manner: the object of introducing the above summation in this paper, is to point out the use of the integral $\int _{-{\frac {\pi }{2}}}^{\frac {\pi }{2}}\varepsilon ^{a\theta }(\cos \theta )^{n}$ , when impossible factors occur in the denominators of the successive terms of a factorial series.

In the 'Exercices de Mathématiques,' Cauchy has proved that if $z$ be a quantity of the form $\zeta (\cos \varphi +i\sin \varphi )$ , and $z\varphi (z)$ continually approach zero as $\zeta$ indefinitely increases whatever be $\varphi$ , then the residue of $\varphi (z)$ is equal to zero, the limits of $\zeta$ being 0 and ($\infty$ ), and those of $\varphi$ , $\pi$ and $-\pi$ . From this theorem he deduces the sums of certain series, which I shall presently consider; but must first give certain results which will be useful in the sequel.

 Since $\int _{0}^{\infty }\varepsilon ^{-ax^{2}}\cos 2xdx={\frac {\sqrt {\pi }}{2{\sqrt {a}}}}\varepsilon ^{-{\frac {1}{a}}}$ $\therefore \ \varepsilon ^{-{\frac {1}{a}}}={\frac {\sqrt {a}}{2{\sqrt {\pi }}}}\int _{-\infty }^{\infty }\varepsilon ^{-{\frac {ax^{3}}{4}}}\cos xdx.$ Again, since $\int _{-\infty }^{\infty }\varepsilon ^{x-ax^{2}}={\frac {\sqrt {\pi }}{\sqrt {a}}}\varepsilon ^{\frac {1}{4a}},$ we find $\varepsilon ^{\frac {1}{a}}={\frac {\sqrt {a}}{2{\sqrt {\pi }}}}\int _{-\infty }^{\infty }\varepsilon ^{x-{\frac {ax^{2}}{4}}}dx,$ whence we have $\varepsilon ^{\frac {1}{a}}-\varepsilon ^{-{\frac {1}{a}}}={\frac {\sqrt {a}}{2{\sqrt {\pi }}}}\int _{-\infty }^{\infty }dx(\varepsilon ^{x}-\cos x)\varepsilon ^{-{\frac {ax^{2}}{4}}}.$  