Transparent Boundary Conditions for the Time-Fractional Heat Equation on Metric Graphs
In this work, we study transparent boundary conditions for the time-fractional heat equation on a metric star graph. This equation is the standard continuum model for subdiffusive heat and mass transport in disordered and porous branched media. Using the Laplace transform to solve the exterior branch problems, we demonstrate that the non-local vertex operator observed by the incoming bond is an Abel-type convolution kernel of order $β/2$, which generalizes the classical half-derivative kernel recovered at $β=1$. Our main result is that the diffusion-coefficient sum rule $κ_1=κ_2+κ_3$, known to eliminate thermal backflow for the classical heat equation, remains unchanged for every fractional order $β$: the transparency of the junction depends only on the network's diffusivities and not on the memory of the transport law. We rigorously prove this and uniqueness for the coupled vertex problem. We confirm our results numerically at the level of the underlying coupled partial differential equation system with an implicit L1 finite-difference scheme. We compare the star-graph solution to a reference solution on an unbranched line with matching diffusivity.