A widely used class of linear operators acting on infinite dimensional spaces are the differential operators on function spaces. Let D be a linear differential operator in on the space mathbf{C^infty} of infinitely differentiable real functions of a real argument t. The eigenvalue equation for D is the differential equation
D f = lambda f
The functions that satisfy this equation are commonly called eigenfunctions of D. For the derivative operator d/dt, an eigenfunction is a function that, when differentiated, yields a constant times the original function. The solution is an exponential function
f(t) = Ae^{lambda t} ,
including when lambda is zero when it becomes a constant function. Eigenfunctions are an essential tool in the solution of differential equations and many other applied and theoretical fields. For instance, the exponential functions are eigenfunctions of the shift operators. This is the basis of Fourier transform methods for solving problems