Small data blow-up for the weakly coupled system of the generalized Tricomi equations with multiple propagation speeds

In the present paper, we study the Cauchy problem for the weakly coupled system of the generalized Tricomi equations with multiple propagation speeds. Our aim of this paper is to prove a small data blow-up result and an upper estimate of lifespan of the problem for a suitable compactly supported initial data in the subcritical and critical cases of the Strauss type. The proof is based on the framework of the argument in the paper (Ikeda et al. in J Diff Equs 267:5165–5201, 2019). One of our new contributions is to construct two families of special solutions to the free equation (see (2.16) or (2.18)) as the test functions and prove their several properties. We emphasize that the system with two different propagation speeds is treated in this paper and the assumption on the initial data is improved from the point-wise positivity to the integral positivity.