  We focus on the global semiconcavity of solutions to first-order
Hamilton--Jacobi equations with state constraints, especially for the
Hamiltonian $H(x, \beta):=|\beta|^p-f(x)$ with $p \in (1, 2]$. We first show
that the solution is locally semiconcave, and the semiconcavity constant at
each point depends on the first time a corresponding minimizing curve emanating
from this point hits the boundary. Then, with appropriate conditions on $Df$,
we prove that for any such minimizing curve, the time it takes to hit the
boundary of the domain is $+\infty$, and as a consequence, the solution is
globally semiconcave. Moreover, the condition on $Df$ is essentially optimal
with examples in one-dimensional space. The proofs employ the Euler-Lagrange
equations and techniques in weak KAM theory.

Analysis of PDEs