# Continuity of a constrained parameterized convex optimization problem

Consider the parameterized optimization problem: \begin{align} \boldsymbol{s}(p)= &\arg \min_{ \boldsymbol{x}} \quad g( \boldsymbol{x})\\ \text{s.t. } & \boldsymbol{A}(p) \textbf{x} = \boldsymbol{b}(p)\\ & \boldsymbol{x}_{min} \preccurlyeq \boldsymbol{x} \preccurlyeq \boldsymbol{x}_{max}, \end{align}

where $$\boldsymbol{A}(p) \in \mathbb{R}^{m\times n}$$, $$\boldsymbol{x} \in \mathbb{R}^{n}$$, $$\boldsymbol{b}(p) \in \mathbb{R}^{n}$$, and $$\preccurlyeq$$ denotes element-wise inequality.

Under the following assumptions:

A.1a. $${g}(\boldsymbol{x})$$ is strictly convex and continuously differentiable.

A.1b. $${g}(\boldsymbol{x})$$ is well behaved $$^*$$

A.2. $$\boldsymbol{A}(p)$$ is full row-rank.

A.3. The elements of $$\boldsymbol{A}(p)$$ and $$\boldsymbol{b}(p)$$ is $$\mathcal{C}^1$$ continuous.

A.4. $$\boldsymbol{A}(p) \textbf{x} = \boldsymbol{b}(p)$$ is feasible for some $$\boldsymbol{x} \in \boldsymbol{x}_{min} \preccurlyeq \boldsymbol{x} \preccurlyeq \boldsymbol{x}_{max}$$

I want to proove continuity of $$\boldsymbol{s}(p)$$.

I think I have a working draft, but could really need some input on making it more precise/verify it.

Step 1 KKT Qualifications and existence of an optimal solution

The following two KKT qualifications are met [1]

1: Linear constraint qualification is met (affine constraints)

2: Second order sufficient condition is met (by A.1a)

With the above KKT qualifications satisfied, if $$\boldsymbol{x}^*$$ is a local optimum, then there exists some KKT multipliers $$\boldsymbol{\lambda}\in \mathbb{R}^{m}$$ and $$\boldsymbol{\mu} \in \mathbb{R}^{2n}$$, such that $$$$\label{KKTPOINKKT} \boldsymbol{R}( \boldsymbol{z},p) = {\left[\begin{matrix}\nabla_{ \boldsymbol{x}^*} {g} ( \boldsymbol{x}^*) + \boldsymbol{\lambda}^{\top} \boldsymbol{A(p)} + \boldsymbol{\mu}^{\top} {\left[\begin{matrix} \boldsymbol{1}\\ \boldsymbol{-1} \end{matrix} \right]} \\ \boldsymbol{A}(p) \boldsymbol{x} - \boldsymbol{b}(p) \\ {\mu}_ 1 h_1( \boldsymbol{x^*}) \\ {\mu}_ 2 h_2( \boldsymbol{x^*}) \\ \vdots \\ {\mu}_{2n} h_{2n}( \boldsymbol{x^*}) \end{matrix} \right]}= \boldsymbol{0}$$$$
for \begin{align} h( \boldsymbol{x}^*) &\preccurlyeq 0,\\ \boldsymbol{\mu}& \succcurlyeq 0 \end{align} where \begin{align} h( \boldsymbol{x})=& {\left[\begin{matrix} \boldsymbol{x}- \boldsymbol{x}_{\text{max}} \\ \boldsymbol{x}_{\text{min}}- \boldsymbol{x} \end{matrix} \right]},\\ \boldsymbol{z} = &{\left[\begin{matrix} \boldsymbol{f} ^T& \boldsymbol{\lambda}^{\top} & \boldsymbol{\mu}^{\top} \end{matrix} \right]^{\top}}. \end{align}

By (A.3) a local optimum $$\boldsymbol{x}^*$$ satisfying the above exists, and by strict convexity (A.1a) this optimum must be unique. This implies $$\boldsymbol{s}(p)= \boldsymbol{x}^*$$.

Step 2 (Show differentiability given that the active set does not change)

Let the the active set $${{\mathbb{A}}}$$ be the set of constraints that are strictly active,
(i.e. $$h_i=0$$, $$\mu_i >0$$ $$\implies h_i \in h_{{\mathbb{A}}}$$ )
Let $$\boldsymbol{R}_{{\mathbb{A}}}( \boldsymbol{z},p)$$, be $$\boldsymbol{R}( \boldsymbol{z},p)$$, only containing the active constraints of $${{\mathbb{A}}}$$.

For a fixed active set $${{\mathbb{A}}}$$, $$\boldsymbol{z}$$ is implicitly defined by $$\boldsymbol{R}_{{\mathbb{A}}}( \boldsymbol{z},p) =0$$: $$$$\label{SENSITIIVITY} \frac{\partial \boldsymbol{z}}{\partial p} = - \left( \frac{\partial \boldsymbol{R} _{\mathbb{A}} }{\partial \boldsymbol{z} } \right)^{-1} \frac{\partial \boldsymbol{R} _{\mathbb{A}}}{\partial p}.$$$$

$$\boldsymbol{R} _{ \mathbb{A} }( \boldsymbol{z},p)$$ is continuously differentiable w.r.t. $$\boldsymbol{z}$$;

$$$$\label{key} \frac{\partial \boldsymbol{R} _{\mathbb{A}} ( \boldsymbol{z},p) }{\partial \boldsymbol{z}}= {\left[\begin{matrix}\nabla^2_{ \boldsymbol{x}} \boldsymbol{g} ( \boldsymbol{x}) & \boldsymbol{A}^{\top}(p) & diag\left( {\left[\begin{matrix} \boldsymbol{1}\\ \boldsymbol{-1} \end{matrix} \right]} _{{\mathbb{A}}} \right) \\ \boldsymbol{A}(p) & \boldsymbol{0} & \boldsymbol{0} \\ diag\left( {\left[\begin{matrix}\boldsymbol{1}\\ \boldsymbol{-1} \end{matrix} \right]} _{{\mathbb{A}}} \right) & \boldsymbol{0} & \boldsymbol{0} \end{matrix} \right]} ,$$$$ and $$\frac{\partial \boldsymbol{R} _{\mathbb{A}} ( \boldsymbol{z},p) }{\partial \boldsymbol{z}}$$ is continuously invertible (by convexity $$\nabla^2_{\boldsymbol{x}} g ( \boldsymbol{x})$$ is positive definite (A.1a), while $$\boldsymbol{A}$$ is full row rank (A.2).

The second term $$$$\label{END} \frac{\partial R( \boldsymbol{z},p)}{\partial p } = {\left[\begin{matrix}\boldsymbol{0} \\ \frac{\partial }{ \partial p } \left( \boldsymbol{A}(p) - \boldsymbol{b}(p) \right)\\ \boldsymbol{0} \end{matrix} \right]}$$$$ is differentiable w.r.t. $$p$$ (A.3)

Therefore, by the implicit function theorem, given a fixed active set $${{\mathbb{A}}}$$, for every pair ($$\bar{ \boldsymbol{z}}, \bar{p}$$) satisfying $$R_{\mathbb{A}}( \bar{ \boldsymbol{z}}, \bar{p} ) =0$$, there, in the neighborhood of $$\bar{p}$$, is a unique function $$\vartheta (\bar{p})$$ such that $$R_{\mathcal{A}}(\vartheta (\bar{p}),\bar{p})=0$$. Therefore, as long as the active set does not change, inserting $$\boldsymbol{z}(p) =\vartheta (\bar{p})$$ shows that $$\boldsymbol{z}$$ (and $$\boldsymbol{s}(p)$$) are continuously differentiable w.r.t. $$p$$.

Step 3 It is now (presumably) established that for a fixed active set, we have $$\mathcal{C}^1$$ continuity. I now want to establish $$\mathcal{C}^0$$ continuity of the whole problem.

$$\frac{\partial \boldsymbol{z}}{\partial p}$$ does not exists in cases where the active set change. However, the left and right-handed derivative exists. Thus $$\boldsymbol{{ z}}$$ (and $$\boldsymbol{s}(p)$$) is piecewise continuously differentiable. From piecewise differentiability, it follows that $$\boldsymbol{s}(p)$$ is continuous.

Some specific questions:

-Does the arguments work out/Is there anything missing? I am especially uncertain on the "active set" argument. (to me it appears to make sense to me).

-Is strict convexity $${g}(\boldsymbol{x})$$ sufficient , or do we need strong convexity?.

-I asserted (A.1b) to ensure that $${g}(\boldsymbol{x})$$ never tend to an infinitum. Is this needed, or does the same follow from (A.1a)?

PS:

This is a continuation of a question problem I asked previously (for which I got a good answer for). I know want to expand it, and be more precise.

• Have you tried using a barrier function to get rid of the inequality constraints? I'm not completely sure about the uniformity of the convergence results there, but if they are somewhat uniform then you could use the continuity result which you already have for the resulting problem and an $\epsilon/3$ argument to get the constrained one. May 28, 2019 at 14:58
• I have tried approximating the inequalities as log-barriers. I assume the idea is that as the log-barrier cost-function approaches a corner-representation, you can show that continuity holds for any "resolution/approximation" of the corner by the epsilon argument. I want to keep the sub-result that we may get discontinuous rates as the active-set changes which I am struggling to do with the epsilon argument. To be honest, I have little experience formulating epsilon type arguments, so if it holds, I would like to keep the "active set" strategy. May 28, 2019 at 16:52
• I meant the following: you want to show that $|s(p) - s(\bar p)|$ is small whenever $|p-\bar p|$ is small. Denote by $s_k(\cdot)$ the solution mapping for the Barrier problems. Then by the corresponding theory $s_k(p) \to s(p)$ as $k\to\infty$, so you divide $|s(p)-s(\bar p)|$ into $|s(p)-s_k(p)|$, $|s_k(p)-s_k(\bar p)|$ and $|s_k(\bar p)-s(\bar p)|$. The hope would be to be able to make these small in a uniform manner in $k$. May 29, 2019 at 11:47
• You wrote " I am especially uncertain on the "active set" argument. (to me it appears to make sense to me). " An epsilon change in $p$ can change the active set. Jun 8, 2019 at 15:19