In this paper we consider the wave equations with power type nonlinearities including timederivatives of unknown functions and their weakly coupled systems. We propose a framework of test function method and give a simple proof of the derivation of sharp upper bound of lifespan of solutions to nonlinear wave equations and their systems. We point out that for respective critical case, we use a family of self-similar solution to the standard wave equation including Gauss's hypergeometric functions which are originally introduced by Zhou [59]. However, our framework is much simpler than that. As a consequence, we found new (p, q)-curve for the system ∂ 2 t u − ∆u = |v| q , ∂ 2 t v − ∆v = |∂ t u| p and lifespan estimate for small solutions for new region. (2010): Primary: 35L05, 35L51, 35B44.
Mathematics Subject Classification