Multi-term fractional linear equations modeling oxygen subdiffusion through capillaries

For (Formula presented.), we analyze a linear integro-differential equation on the space-time cylinder (Formula presented.) in the unknown (Formula presented.) (Formula presented.) where (Formula presented.) are the Caputo fractional derivatives, (Formula presented.) with (Formula presented.), (Formula presented.) are uniform elliptic operators with time-dependent smooth coefficients, (Formula presented.) is a summable convolution kernel, and f is an external force. Particular cases of this equation are the recently proposed advanced models of oxygen transport through capillaries. Under suitable conditions on the given data, the global classical solvability of the associated initial-boundary value problems is addressed. To this end, a special technique is needed, adapting the concept of a regularizer from the theory of parabolic equations. This allows us to remove the usual assumption about the nonnegativity of the kernel representing fractional derivatives. The problem is also investigated from the numerical point of view.