Clairaut's equation

In mathematics, more specifically in mathematical analysis, Clairaut's equation (or Clairaut equation) is a differential equation of the form

where f is continuously differentiable. It is a particular case of the Lagrange differential equation.

This equation is named after the French mathematician Alexis Clairaut, who introduced it in 1734.

To solve Clairaut's equation, we differentiate with respect to x, yielding

so

Hence, either

or

In the former case, C = dy/dx for some constant C. Substituting this into the Clairaut's equation, we have the family of straight line functions given by

the so-called general solution of Clairaut's equation.

The latter case,

defines only one solution y(x), the so-called singular solution, whose graph is the envelope of the graphs of the general solutions. The singular solution is usually represented using parametric notation, as (x(p), y(p)), where p = dy/dx.

The following curves represent the solutions to two Clairaut's equations:

In each case, the general solutions are depicted in black while the singular solution is in violet.

By extension, a first-order partial differential equation of the form

is also known as Clairaut's equation.

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