sentences:  If the reaction term vanishes, then the equation represents a pure diffusion process.The corresponding equation is Fick's second law.The choice yields Fisher's equation that was originally used to describe the spreading of biological populations, the Newell–Whitehead-Segel equation with to describe Rayleigh–Bénard convection, the more general Zeldovich equation with and that arises in combustion theory, and its particular degenerate case with that is sometimes referred to as the Zeldovich equation as well.