HP 48gII - Manual (Page 506)

Page 16-28
Thus, the first three terms of the function are:
f(t) ≈ 1/3 – (4/π 2 ) ⋅cos (π⋅t)+(2/π)⋅sin (π⋅t).
A graphical comparison of the original function with the Fourier expansion
using these three terms shows that the fitting is acceptable for t < 1, or
thereabouts. But, then, again, we stipulated that T/2 = 1. Therefore, the
fitting is valid only between –1 < t < 1.
Function FOURIER
An alternative way to define a Fourier series is by using complex numbers as
follows:
∑
+∞
−∞ =
⋅ =
n
n T
t in
c t f ),
2
exp( ) (
π
where