The equation in 11.9 should be
∂ 2 u ∂ x 2 + ∂ 2 u ∂ y 2 = 0 {\displaystyle {\frac {\partial ^{2}u}{\partial x^{2}}}+{\frac {\partial ^{2}u}{\partial y^{2}}}=0} .
Return to errata.
