An Etymological Dictionary of Astronomy and Astrophysics
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One of the coefficients a_n or b_n of cos (nx) and sin (nx) respectively in the → Fourier series representation of a function. They are expressed by:
a_n = (1/π) ∫f(x) cos nx dx, for n≥ 0, summed over 0 to 2π
b_n = (1/π) ∫f(x) sin nx dx, for n≥ 1, summed over 0 to 2π.

→ Fourier analysis; → series.