The Heaviside expansion theorem is widely known and used in circuit theory. It is, in fact, a mathematical result with a long history. Its discovery forms a chapter in the history of differential equations and it was arrived at independently at least three times before Heaviside first announced it. It may be regarded as being implicit in the work of Euler, Lagrange and Cauchy, but it is also given almost directly by Cauchy, Lobatto and Boole, although Heaviside's later discovery was most probably independent.