The equation should be:
∂ 2 u t r ∂ x 2 + ∂ 2 u s s ∂ x 2 + ∂ 2 u t r ∂ y 2 + ∂ 2 u s s ∂ y 2 = ∂ u t r ∂ t + ∂ u s s ∂ t . {\displaystyle {\frac {\partial ^{2}u_{tr}}{\partial x^{2}}}+{\frac {\partial ^{2}u_{ss}}{\partial x^{2}}}+{\frac {\partial ^{2}u_{tr}}{\partial y^{2}}}+{\frac {\partial ^{2}u_{ss}}{\partial y^{2}}}={\frac {\partial u_{tr}}{\partial t}}+{\frac {\partial u_{ss}}{\partial t}}.}
Return to errata.
