Ridge regression (also known as Tikhonov regularization) shrinks the regression coefficients by adding a quadratic penalty term to the optimization problem. $\underset{\beta}{\text{minimize}} ~~ \frac{1}{2} || y - X \beta ||_2^2 + \lambda ||\beta||_2^2$ with $$X \in \mathbb{R}^{m \times n}$$, $$y \in \mathbb{R}^{m}$$, $$\beta \in \mathbb{R}^n$$ and $$0 < \lambda \in \mathbb{R}$$.

It is well known that $$\beta$$ can be estimated by the following formula. $\hat{\beta} = (X^\top X + \lambda I)^{-1} X^\top y$ here $$I \in R^{n, n}$$ refers to the identity matrix.

The optimization problem can be rewritten into a quadratic optimization problem $\begin{array}{rl} \underset{(\beta, \gamma)}{\text{minimize}} & \frac{1}{2} \gamma^\top \gamma + \lambda \beta^\top \beta \\ \text{subject to} & y - X \beta = \gamma \end{array}$ with $$\gamma \in \mathbb{R}^n$$.

In R the packages glmnet and MASS provide functionality for ridge regression.

y <- longley[,1]
x <- as.matrix(longley[,-1])

## Estimation

library(slam)
Sys.setenv(ROI_LOAD_PLUGINS = FALSE)
library(ROI)
library(ROI.plugin.qpoases)

dbind <- function(...) {
.dbind <- function(x, y) {
A <- simple_triplet_zero_matrix(NROW(x), NCOL(y))
B <- simple_triplet_zero_matrix(NROW(y), NCOL(x))
rbind(cbind(x, A), cbind(B, y))
}
Reduce(.dbind, list(...))
}
qp_ridge <- function(x, y, lambda) {
stdm <- simple_triplet_diag_matrix
m <- NROW(x); n <- NCOL(x)
Q0 <- dbind(stdm(2 * lambda, n), stdm(1, m))
a0 <- c(b = double(n), g = double(m))
op <- OP(objective = Q_objective(Q = Q0, L = a0))
A1 <- cbind(x, stdm(1, m))
constraints(op) <- L_constraint(A1, eq(m), y)
bounds(op) <- V_bound(ld = -Inf, nobj = ncol(Q0))
op
}

op <- qp_ridge(x, y, 0)
(qp0 <- ROI_solve(op, "qpoases"))
## Optimal solution found.
## The objective value is: 6.616251e+00
cbind(round(coef(lm.fit(x, y)), 3), round(head(solution(qp0), ncol(x)), 3))
##                [,1]   [,2]
## GNP           0.217  0.217
## Unemployed    0.021  0.021
## Armed.Forces  0.004  0.004
## Population   -1.700 -1.700
## Year          0.117  0.117
## Employed     -0.311 -0.311
op <- qp_ridge(x, y, 10)
(qp1 <- ROI_solve(op, "qpoases"))
## Optimal solution found.
## The objective value is: 1.087148e+01
head(solution(qp1), NCOL(x))
##   0.11039939  0.01231982  0.01120254 -0.24179694  0.03957795  0.04689382