(That "u" defined in this way satisfies equation (2) is easy enough to check, one needs to integrate separately on the regions above and below the line "x" = "t" and use integration by parts.
(Este "u" definido de esta forma satisface la ecuación (2) que es fácil de comprobar, al integrar separadamente sobre las regiones arriba y abajo de la recta "x" = "t" y usando la integración por partes.