Figure 1: Illustration when $n=p=3$ of primal and dual admissible sets $C$ and $D$. Solution to (1) can be determined by a projection of $y$. Furthermore, if $A^{-1}$ exists, then the primal solution can be expressed as a function of $y$.

Let $A: \mathbb{R}^{p} \rightarrow \mathbb{R}^{n}$ and $y \in \mathbb{R}^{p}$. Consider the Tikhonov regularization

$\newcommand{\norm}[1]{\left\lVert#1\right\rVert} $$\min_{x \in \mathbb{R}^{n}} \frac{1}{2} \norm{Ax - y}_{2}^{2} + \alpha \norm{x}_{1}. \label{eq:min-problem}$$$

This type of penalized regression is called Lasso; see Tibshirani’s original paper [1].

In this post, we first derive the dual problem, then show that the solution $x^{*}$ can be determined with the help of a projection operator. Under the assumption that $A^{-1}$ exists, we can further express the solution $x^{*}$ with the help of a mapping $\left(A^{T}\right)^{-1}$ from the dual space as a function of $y$; see Figure 1.

Some nice and non-obvious properties of the solution $x^{*}$ follow from the geometry of the dual formulation. For example that the Lasso solution is non-expansive as a function of $y$. This is not obvious and would probably be hard to show without the dual formulation.

## Formulation of the dual problem

To derive the dual problem, we can introduce a dummy variable $z \in \mathbb{R}^{n}$

$$$\nonumber z = Ax$$$

and reformulate the minimization problem in \eqref{eq:min-problem} as a constrained problem

\begin{align*} \label{eq:primal-problem} \tag{P} &\underset{z \in \mathbb{R}^{n}, x \in \mathbb{R}^{p}}{\text{minimize}} \quad \frac{1}{2} \norm{y - z}_{2}^{2} + \alpha \norm{x}_{1} \\ &\text{subject to} \quad z = Ax \end{align*}

Then we can construct the Lagrangian by introducing the dual variable $p \in \mathbb{R}^{n}$ (containing $n$ Lagrange multipliers)

$$$\nonumber L(x, z, p) = \frac{1}{2} \norm{y - z}_{2}^{2} + \alpha \norm{x}_{1} + p^{T} (z - Ax)$$$

The dual objective function is

$$$\nonumber g(p) = \min_{z \in \mathbb{R}^{n}, x \in \mathbb{R}^{p}} \left\lbrace \frac{1}{2} \norm{y - z}_{2}^{2} + \alpha \norm{x}_{1} + p^{T} (z - Ax) \right\rbrace$$$

We can split the terms depending on $z$ and $x$ and minimize each part separately

\begin{align}\nonumber g(p) &= \min_{z \in \mathbb{R}^{n}, x \in \mathbb{R}^{p}} \left\lbrace \frac{1}{2} \norm{y}_{2}^{2} - y^{T}z + \frac{1}{2} \norm{z}_{2}^{2} + p^{T}z + \alpha \norm{x}_{1} - p^{T}Ax \right\rbrace \\[.3em] &= \min_{z \in \mathbb{R}^{n}}\nonumber \left\lbrace \frac{1}{2} \norm{y}_{2}^{2} - (y - p)^{T}z + \frac{1}{2} \norm{z}_{2}^{2} \right\rbrace + \max_{x \in \mathbb{R}^{p}} \left\lbrace \alpha \norm{x}_{1} - (A^{T}p)^{T}x \right\rbrace \end{align}

We can use the stationarity condition, which says that at the optimal point, the subgradient of $L(x, z, p)$ with respect to $x$ and $z$ must contain 0.

For the first part, since $L(x, z, p)$ is differentiable in $z$, the subgradient with respect to $z$ equals the gradient. By taking $\frac{\partial}{\partial z} L(x, z, p)$ and setting it to $0$, we get the stationarity condition

$$$\nonumber z = y - p^{*}$$$

Plugging this into the first part, we get

\begin{align}\nonumber &\frac{1}{2}\norm{y}_{2}^{2} - (y - p^{*})^{T}(y - p^{*}) + \frac{1}{2}\norm{y - p^{*}}_{2}^{2} \\[.3em] &= \frac{1}{2}\norm{y}_{2}^{2}\nonumber - \frac{1}{2}\norm{y - p^{*}}_{2}^{2} \end{align}

For the second part, because $\alpha \norm{x}_1$ is a non-differentiable function of $x$, we need to compute the subdifferential $\partial (\alpha \norm{x}_1)$.

By using the rules for the subgradient of the maximum we can derive, see [2], that the $\partial \norm{x}_{1}$ can be expressed as

$$$\nonumber \partial (\norm{x}_{1}) = \lbrace g : \norm{g}_{\infty} \leq 1, g^{T} x = \norm{x}_{1} \rbrace$$$

and using the rule for scalar multiplication

$$$\nonumber \partial (\alpha\norm{x}_{1}) = \lbrace g : \norm{g}_{\infty} \leq \alpha, g^{T} x = \alpha\norm{x}_{1} \rbrace$$$

Thus, we get the stationarity condition

$$$\nonumber g = A^{T}p \in \partial \alpha \norm{x}_{1}$$$

when

\begin{align} \nonumber \norm{A^{T}p}_{\infty} \leq \alpha \\[.3em] \alpha \norm{x}_{1} - (A^{T}p)^{T}x = 0\nonumber \end{align}

Therefore, the dual problem is

\begin{align*} \label{eq:dual-problem} \tag{D} &\max_{p \in \mathbb{R}^{n}} \frac{1}{2}\norm{y}_{2}^{2} - \frac{1}{2}\norm{y - p}_{2}^{2} \\[.3em] &\text{subject to} \norm{A^{T}p}_{\infty} \leq \alpha \end{align*}

## Solution of the dual problem

The solution $p^{*}$ of the dual problem can be determined with the help of a projection operator.

Looking at the dual problem formulation \eqref{eq:dual-problem}, we see that we can omit $\frac{1}{2}\norm{y}_{2}^{2}$, since this is constant. Then if we multiply it by 2 and flip the sign, we get an equivalent dual problem

\begin{align*} \label{eq:dual-problem-prime} \tag{D'} &\min_{p \in \mathbb{R}^{n}} \norm{y - p}_{2}^{2} \\[.3em] &\text{subject to} \norm{A^{T}p}_{\infty} \leq \alpha \end{align*}

A projection operator $P_{C}$ can be defined as

\DeclareMathOperator*{\argmin}{arg\,min} \begin{align}\nonumber P_{C} \text{: } & \mathbb{R}^{n} \rightarrow C \\[.3em] & y \mapsto P_{C}(y) := \argmin_{p \in C} \norm{y - p}_{2} \nonumber \end{align}

where $C \subset \mathbb{R}^{n}$ is closed and convex set. Looking at \eqref{eq:dual-problem-prime}, we see that $p^{*}$ is a projection

$$$\nonumber p^{*} = P_{C}(y)$$$

where $C$ is equal to

$$$\nonumber C = \lbrace p \in \mathbb{R}^{n} : \norm{A^{T}p}_{\infty} \leq \alpha \rbrace$$$

We notice that $C$ is indeed closed and convex. For example if $n=p=2$, and let

$$$\nonumber A^{T} = \begin{pmatrix} a_{11} & a_{21}\\ a_{12} & a_{22} \end{pmatrix}$$$

then

$$$\nonumber C = \lbrace -\alpha \leq a_{11} p_{1} + a_{12} p_{2} \leq \alpha \rbrace \cap \lbrace -\alpha \leq a_{21} p_{1} + a_{22} p_{2} \leq \alpha \rbrace$$$

Or we can express $C$ as

$$$\nonumber C = (A^{T})^{-1}(D)$$$

where $D$ is equal to

$$$\nonumber D = \lbrace d \in \mathbb{R}^{p} : \norm{d}_{\infty} \leq \alpha \rbrace$$$

Again for $p=2$

$$$\nonumber D = \lbrace -\alpha \leq d_{1} \leq \alpha \rbrace \cap \lbrace -\alpha \leq d_{2} \leq \alpha \rbrace$$$
Figure 2: Illustration when $n=p=2$ of primal and dual admissible sets $C$ and $D$.

## Solution of the primal problem

From optimality condition of the dual problem, we can derive the primal solution under the assumption that $A^{-1}$ exists.

During the formulation of the dual problem, we introduced the dummy variable $z = Ax$ and derived the stationary condition $z = y - p^{*}$. From this, we get that every solution $x^{*}$ of \eqref{eq:primal-problem} should satisfy

$$$\nonumber A x^{*} = y - p^{*}$$$

where $p^{*}$ is a solution of the dual problem \eqref{eq:dual-problem}. Therefore, if $A^{-1}$ exists, the primal solution is

$$$\nonumber x^{*} = A^{-1} \left( y - P_{C}(y) \right)$$$

We notice that, since $P_{C}$ is a projection onto convex set $C$, it follows that $x^{*}$ is also non-expansive as a function of $y$; see Figure 3(a) in contrast to Figure 3(b) showing projection onto non-convex set $N$. This is not obvious and would probably be hard to show without the dual formulation.

(a) Convex set $C$. (b) Non-convex set $N$.
Figure 3: Lasso solution $x^{*}$ is non-expansive as a function of $y$.

## References

[1] Tibshirani, R. (1996). Regression shrinkage and selection via the lasso. J. Royal. Statist. Soc B., Vol. 58, No. 1, pages 267-288). <a target=”_blank” href=”https://webdoc.agsci.colostate.edu/koontz/arec-econ535/papers/Tibshirani%20(JRSS-B%201996).pdf</a>

[2] S. Boyd and L. Vandenberghe, “Subgradients Notes”, (2008), Stanford University https://see.stanford.edu/materials/lsocoee364b/01-subgradients_notes.pdf