A remark on principal‐value integrals

Many physical problems lead to integrals that can be evaluated by contour integration, and sometimes there are poles on the contour. In practice, it usually is appropriate in such cases to express the result as a principal‐value integral. Most textbooks treat each problem involving principal values individually by considering an indented contour and taking limits. We want to call the attention of users of the calculus of residues to a theorem which, although known, seems not to be well‐known.