Louise Petrén is an interesting person in the history of Swedish mathematics. In her PhD thesis defended in Lund in 1911, she extended to higher-order equations Laplace’s method of integration of second-order linear hyperbolic equations with two independent variables. It is interesting to consider her results from point of view of invariants of differential equations and compare with the theory of the Laplace invariants. However, L. Petrén’s research was not known until recently among mathematicians working in group analysis. The aim of our talk is to introduce Louise Petrén as a person and to discuss her generalization of Laplace’s method. Lars Haikola will contribute with a family background of Louise Petrén.