# D'Alembert formula

A formula expressing the solution of the Cauchy problem for the wave equation with one spatial variable. Let the given functions , belong, respectively, to the spaces and , and let be continuous together with the first derivative with respect to in the half-plane . Then the classical solution in of the Cauchy problem (1) (2)

is expressed by d'Alembert's formula:  If the functions and are given and satisfy the above smoothness conditions on the interval , and if satisfies it in the triangle then d'Alembert's formula gives the unique solution of the problem (1), (2) in . The requirements on the given functions may be weakened if one is interested in solutions in a certain generalized sense. For instance, it follows from d'Alembert's formula that if is integrable with respect to any triangle , if is locally integrable and if is continuous, the weak solution of Cauchy's problem (1), (2) may be defined as a uniform limit (in any ) of classical solutions with smooth data and is also expressed by d'Alembert's formula.

The formula was named after J. d'Alembert (1747).