Blow-up criteria for the semilinear parabolic equations driven by mixed local-nonlocal operators

The main goal of this paper is to establish necessary and sufficient conditions for the nonexistence of a global solution to the semilinear heat equation with a mixed local--nonlocal operator -Δ+ (-Δ)^σ, under a general time-dependent nonlinearity. Our results complement the recent work of Carhuas-Torre et al. [ArXiv, (2025), arXiv:2505.20401], in which the authors provide sufficient conditions for the existence and nonexistence of global solutions. In particular, our results recover the critical Fujita exponent for time-independent power-type nonlinearities, as obtained by Biagi et al. [Bull. London Math. Soc. (2024), 1--20] and Del Pezzo et al. [Nonlinear Anal. 255 (2025), 113761].