The integrals ∫ x 0 sin u / u du , ∫ x ∞ cos u / u du , ∫ ∞ -x e - u u / du called the sine-integral, cosine-integral, and exponential integral, were used by Schlömilch to express the values of several more complicated integrals, and denoted by him thus,—Si x , Ci x , Ei x ; the last function, however, is for all real values of x only another form of the logarithm-integral, the relation being Ei x =li e x . These functions have since been shown to be the key to a very large class of definite integrals, and several hundreds have been evaluated in terms of them by Schlömilch, De Haan, &c., so that for some time they have been considered primary functions of the integral calculus, and forms reduced to dependence on them have been regarded as known.
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