D'Alembert's formula


In mathematics, and specifically partial differential equations, d'Alembert's formula is the general solution to the one-dimensional wave equation
.
The solution depends on the initial conditions at : and.
It consists of separate terms for the initial conditions and :
It is named after the mathematician Jean le Rond d'Alembert, who derived it in 1747 as a solution to the problem of a vibrating string.

Details

The characteristics of the PDE are, so we can use the change of variables to transform the PDE to. The general solution of this PDE is where and are functions. Back in coordinates,
This solution can be interpreted as two waves with constant velocity moving in opposite directions along the x-axis.
Now consider this solution with the Cauchy data.
Using we get.
Using we get.
We can integrate the last equation to get
Now we can solve this system of equations to get
Now, using
d'Alembert's formula becomes:

Generalization for inhomogeneous canonical hyperbolic differential equations

The general form of an inhomogeneous canonical hyperbolic type differential equation takes the form of:
for.
All second order differential equations with constant coefficients can be transformed into their respective canonic forms. This equation is one of these three cases: Elliptic partial differential equation, Parabolic partial differential equation and Hyperbolic partial differential equation.
The only difference between a homogeneous and an inhomogeneous differential equation is that in the homogeneous form we only allow 0 to stand on the right side, while the inhomogeneous one is much more general, as in could be any function as long as it's continuous and can be continuously differentiated twice.
The solution of the above equation is given by the formula:
If, the first part disappears, if, the second part disappears, and if, the third part disappears from the solution, since integrating the 0-function between any two bounds always results in 0.
This means, that the homogeneous equation gives back our original formula for the case of.