Ziming Li Fritz Schwarz Serguei P. Tsarev Factoring systems of linear PDEs with finite-dimensional solution spaces. 443-471 2003 36 J. Symb. Comput. 3-4 http://dx.doi.org/10.1016/S0747-7171(03)00090-7 db/journals/jsc/jsc36.html#LiST03