FourierCosSeries

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

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

Examples

Compute a 3rd-order Fourier cosine series:

FourierCosSeries[t^2, t, 3]
(* Pi^2/3 - 4 Cos[t] + Cos[2 t] - 4/9 Cos[3 t] *)

Please visit the official Wolfram Language Reference for more details.