This paper develops an abstract theory for subdifferential operators to give existence and uniqueness of solutions to the initial‐boundary problem for the nonlinear diffusion equation in an unbounded domain
normalΩ⊂RN(
N∈double-struckN), written as
∂u∂t+(−Δ+1)β(u)=ginΩ×(0,T),
which represents the porous media, the fast diffusion equations, etc, where β is a single‐valued maximal monotone function on
double-struckR, and T>0. In Kurima and Yokota (J Differential Equations 2017; 263:2024‐2050 and Adv Math Sci Appl 2017; 26:221‐242) existence and uniqueness of solutions for were directly proved under a growth condition for β even though the Stefan problem was excluded from examples of . This paper completely removes the growth condition for β by confirming Cauchy's criterion for solutions of the following approximate problem ε with approximate parameter ε>0:
∂uε∂t+(−Δ+1)(ε(−Δ+1)uε+β(uε)+πε(uε))=ginΩ×(0,T),
which is called the Cahn‐Hilliard system, even if
normalΩ⊂RN(
N∈double-struckN) is an unbounded domain. Moreover, it can be seen that the Stefan problem excluded from Kurima and Yokota (J Differential Equations 2017; 263:2024‐2050 and Adv Math Sci Appl 2017; 26:221‐242) is covered in the framework of this paper.