mgcv. It is a service routine for
gam which splits a GAM formula into two parts: one representing the smooth part of the
model and the other representing the regular parametric part of the model (including any intercept term).
The processing of
the smooth terms in the model also returns the number of knots for each smooth term, and whether
or not the smooth term is to be treated as a pure regression spline (i.e. one without a penalty).
For further information on usuage see code for gam.Wood (2000) Modelling and Smoothing Parameter Estimation with Multiple Quadratic Penalties. JRSSB 62(2):413-428
gam mgcv