Because the derivative of a function `y = f( x)` is itself a function `y′ = f′( x),` you can take the derivative of `f′( x),` which is generally referred to as the second derivative of `f(x)` and written `f( x)` or `f^ 2 ( x).` This differentiation process can be continued to find the third, fourth, and successive derivatives of `f( x),` which are called higher order derivatives of `f( x).` Because the prime notation for derivatives would eventually become somewhat messy, it is preferable to use the numerical notation `f( text()^n )( x) = y( text()^n )` to denote the `nth` derivative of `f( x).`