A comparison principle for an American option on several assets: index and spread options. Laurence, Peter; Stredulinsky, Edward — 2003 Electronic Journal of Differential Equations (EJDE) [electronic only]
On quasiconvex equimeasurable rearrangement, a counterexample and an example. Peter Laurence; Edward Stredulinsky — 1994 Journal für die reine und angewandte Mathematik