Victor A. Galaktionov 
Geometric Sturmian Theory of Nonlinear Parabolic Equations and Applications [PDF ebook] 

Unlike the classical Sturm theorems on the zeros of solutions of second-order ODEs, Sturm’s evolution zero set analysis for parabolic PDEs did not attract much attention in the 19th century, and, in fact, it was lost or forgotten for almost a century. Briefly revived by P, lya in the 1930’s and rediscovered in part several times since, it was not un