On the solution existence for collocation discretizations of time-fractional subdiffusion equations

Time-fractional parabolic equations with a Caputo time derivative of order $\alpha \in (0,1)$ are discretized in time using continuous collocation methods. For such discretizations, we give sufficient conditions for existence and uniqueness of their solutions. Two approaches are explored: the Lax-Milgram Theorem and the eigenfunction expansion. The resulting sufficient conditions, which involve certain $m\times m$ matrices (where $m$ is the order of the collocation scheme), are verified both analytically, for all $m\ge 1$ and all sets of collocation points, and computationally, for all $m\le 20$. The semilinear case is also addressed.