Some remarks on the null condition for nonlinear elastodynamic system (Q1848443)

This short note shows the equivalence among the null condition introduced by \textit{T. C. Sideris} [Ann. Math. 151, No. 2, 849-874 (2000; Zbl 0957.35126)] and the null condition introduced by \textit{R. Agemi} [Invent. Math. 142, No. 2, 225-250 (2000; Zbl 1095.35009)] to study the global existence in nonlinear elasticity. The null condition is a special nonresonance condition to avoid blow-up of suitable small initial data. The author thinks that the method here proposed to derive the null condition is simpler than the one used by Agemi.