Pointwise and viscosity solutions for the limit of a two phase parabolic singular perturbation problem Luis CaffarelliC. LedermanN. Wolanski viscosityparabolic singular perturbation problem Indiana University Mathematics Journal 1997 text pdf 10.1512/iumj.1997.46.1413 10.1512/iumj.1997.46.1413 en Indiana Univ. Math. J. 46 (1997) 719 - 740 state-of-the-art mathematics http://iumj.org/access/