**Graph-Constrained Regression with Enhanced Regularization
Parameters**

Performs graph-constrained regularization in which regularization parameters are selected with the use of a known fact of equivalence between penalized regression and Linear Mixed Model solutions. Provides implementation of three regression methods where graph-constraints among coefficients are accounted for.

`riPEERc`

(ridgified Partially Empirical Eigenvectors for Regression with constant) method utilizes additional Ridge term to handle the non-invertibility of a graph Laplacian matrix.`vrPEER`

(variable reducted PEER) method performs variable-reduction procedure to handle the non-invertibility of a graph Laplacian matrix.`riPEER`

(ridgified Partially Empirical Eigenvectors for Regression) method employs a penalty term being a linear combination of graph-originated and ridge-originated penalty terms, whose two regularization parameters are ML estimators from corresponding Linear Mixed Model solution.

Notably, in `riPEER`

method a graph-originated penalty
term allows imposing similarity between coefficients based on graph
information given whereas additional ridge-originated penalty term
facilitates parameters estimation: it reduces computational issues
arising from singularity in a graph- originated penalty matrix and
yields plausible results in situations when graph information is not
informative or when it is unclear whether connectivities represented by
a graph reflect similarities among corresponding coefficients.