Integration is the basic operation in integral calculus. Numerical integration of the arc length integral is usually very efficient. There have been a number of approximations for the exponential integral function. This is an implicit solution which involves a nonelementary integral. The foundations of differential and integral calculus had been laid. This concept was the predecessor to the modern method of integration used in integral calculus. The equivalence of Faraday's law in differential and integral form follows likewise. This reduces the problem of computing a double integral to computing one-dimensional integrals. Schmidt operators that are especially important in the study of integral equations. The process of finding the value of an integral is called "integration". That's where the integral and derivative terms play their part. For the integral around , define as . This is called an expansion as a trigonometric integral, or a Fourier integral expansion. It has a simple expression in terms of the Fresnel integral. But this is a fact that is beyond the reach of the Riemann integral.