We show that Lord Kelvin's method of images is a way to prove generation theorems for semigroups of operators. To this end we exhibit three examples: a more direct semigroup-theoretic treatment of abstract delay differential equations, a new derivation of the form of the McKendrick semigroup, and a generation theorem
The general ideaLord Kelvin's method of images is an ingenious way of solving problems involving boundary conditions, see e.g. [4,7,11,14,18,19,24] and references given there. The idea of employing the method to prove generation theorems for semigroups of operators goes apparently back to W. Feller who constructed the semigroup of the minimal (and the reflected) Brownian motion on R + by noting that the space of odd (even) functions is left invariant by the semigroup of the unrestricted Brownian motion on R, see [12], pp. 341-343, comp. [3]. These semigroups are generated by the one-dimensional Laplacian with Dirichlet and Neumann boundary conditions at the origin, respectively. A question whether a similar construction can be carried out also in the case of more general boundary conditions, seems to have been opened for Communicated