∂ 2 f ∂ x 2 − 1 v 2 ∂ 2 f ∂ t 2 = 0 {\displaystyle {\frac {\partial ^{2}f}{\partial x^{2}}}-{\frac {1}{v^{2}}}{\frac {{\partial }^{2}f}{{\partial t}^{2}}}=0}
. . .