On critical exponents for weak solutions of the Cauchy problem for a non-linear equation of composite type

We consider the Cauchy problem for a model partial differential equation of third order with non-linearity of the form |u|q, where u = u(x, t) for x ∈ R3 and t ≥ 0. We construct a fundamental solution for the linear part of the equation and use it to obtain analogues of Green’s third formula for elliptic operators, first in a bounded domain and then in unbounded domains. We derive an integral equation for classical solutions of the Cauchy problem. A separate study of this equation yields that it has a unique inextensible-in-time solution in weighted spaces of bounded and continuous functions. We prove that every solution of the integral equation is a local-in-time weak solution of the Cauchy problem provided that q > 3. When q ∈ (1, 3], we use Pokhozhaev’s non-linear capacity method to show that the Cauchy problem has no local-in-time weak solutions for a large class of initial functions. When q ∈ (3, 4], this method enables us to prove that the Cauchy problem has no global-in-time weak solutions for a large class of initial functions. © 2021 Russian Academy of Sciences (DoM) and London Mathematical Society