Iteratively projects X off each grouping factor until convergence. Correia (2016) shows this converges geometrically for any finite number of factors; in practice 10-30 iterations suffice for the tolerances used below.
.absorb_demean_multi(X, groups, tol = 1e-10, max_iter = 200L)Within-transformed X.
Numeric vector or matrix.
A list of factors (from .absorb_resolve()).
Numeric: convergence tolerance on max absolute change.
Integer: hard iteration cap.