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In this contribution, we investigate an initial-boundary value problem for a fractional diffusion equation with Caputo fractional derivative of space-dependent variable order where the coefficients are dependent on spatial and time variables. We consider a bounded Lipschitz domain and a homogeneous Dirichlet boundary condition. Variable-order fractional differential operators originate in anomalous diffusion modelling. Using the strongly positive definiteness of the governing kernel, we establish the existence of a unique weak solution in u ∈ L ∞ ( ( 0 , T ) , H 0 1 ( Ω ) ) $u\in \operatorname{L}^{\infty } ((0,T),\operatorname{H}^{1}_{0}( \Omega ) )$ to the problem if the initial data belongs to H 0 1 ( Ω ) $\operatorname{H}^{1}_{0}(\Omega )$ . We show that the solution belongs to C ( [ 0 , T ] , H 0 1 ( Ω ) ∗ ) $\operatorname{C} ([0,T],{\operatorname{H}^{1}_{0}(\Omega )}^{*} )$ in the case of a Caputo fractional derivative of constant order. We generalise a fundamental identity for integro-differential operators of the form d d t ( k ∗ v ) ( t ) $\frac{\mathrm{d}}{\mathrm{d}t} (k\ast v)(t)$ to a convolution kernel that is also space-dependent and employ this result when searching for more regular solutions. We also discuss the situation that the domain consists of separated subdomains.

Existence of a unique weak solution to a non-autonomous time-fractional diffusion equation with space-dependent variable order