Consider the FARIMA model
$$(1-B)^d Y_t = ar_1 X_{t-1} + ... + ar_p X_{t-p}+ma_1 e_{t-1}+...+ma_q e_{t-q}+e_t,$$
where \(e_t\) are the innovations and where \(X_t=(1-B)^d Y_t\).
\(d\) is the fractional differencing
coefficient.
The fractional differencing operator \((1-B)^d\) can alternatively be expressed
as an infinite coefficient series, so that
$$(1-B)^d=\sum_{l=0}^{\infty}b_l B^k,$$
where \(B\) is the backshift operator and where \(b_l\), \(l=0,1,2,...\),
are the coefficients. Note that \(b_0=1\) by definition.
The function returns the series of coefficients \(\{b_l, l =0,1,2,...\}\).