f_ppr

<p>Computes the loss of a smoother fit to projected covariates and corresponding response values.
  Only used within optimisation of projection. Not intended for alternative use.</p>

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Implements the method of Hofmeyr, D.P. (2019) <10.1109/TPAMI.2019.2930501> for fast evaluation of univariate kernel smoothers based on recursive computations.
Applications to the basic problems of density and regression function estimation are provided, as well as some projection pursuit methods
for which the objective is based on non-parametric functionals of the projected density, or conditional density of a response given projected
covariates.

David Hofmeyr

f_ppr: Projection index for projection pursuit regression

Description

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