where L τ and L ψ denote the second-order differentiation matrices with respect to log τ and ψ, respectively, and λ τ and λ ψ are the corresponding regularization parameters that control the smoothness imposed with respect to the relaxation timescale and experimental state. While this work focuses on a single (scalar) experimental state variable, the framework can be readily extended to multiple experimental state variables, which can be represented as vectors .