We now determine integrals over unbounded intervals or unbounded functions. Such integrals are the basis for integral transformations such as the Laplace or Fourier transformation. The essential tool for determining such improper integrals is the concept of the limit: We set a fictitious limit d and calculate a definite integral \(I=I(d)\) depending on d and then consider whether, for example, the limit \(\lim _{d \to \pm \infty } I(d)\) exists.