FourierSeries

FourierSeries[expr, t, n] gives the nth-order Fourier series expansion of expr in t.

FourierSeries[expr, {t1, t2, …}, {n1, n2, …}] gives the multidimensional Fourier series.

Examples

Compute a Fourier series:

FourierSeries[t, t, 3]

Fourier series of a square wave:

FourierSeries[Sign[Sin[t]], t, 5]

Please visit the official Wolfram Language Reference for more details.