  We establish precise asymptotic expansions for solutions to semilinear wave
equations with power-type nonlinearities on asymptotically flat spacetimes. For
cubic nonlinearities $a(t,x)\phi^3$, we prove $\phi(t, x) = 2c t^{-2} +
O(t^{-3+})$ in compact spatial regions, with $c$ computable. For $a(t,x)\phi^p$
with $p \geq 4$, we show $\phi(t, x) = d t^{-3} + O(t^{-4+})$, extending
Price's law to the nonlinear setting. Our approach combines radiation field
analysis with a generalized low-energy resolvent expansion, providing a bridge
between spectral and physical space methods. These results sharpen previous
decay estimates and yield complete asymptotics across the entire spacetime,
including black hole backgrounds.
