Sequential linear-quadratic programming
# Sequential linear-quadratic programming

Sequential linear-quadratic programming (SLQP) is an iterative method for nonlinear optimization problems where objective function and constraints are twice continuously differentiable. Similarly to sequential quadratic programming (SQP), SLQP proceeds by solving a sequence of optimization subproblems. The difference between the two approaches is that:

* in SQP, each subproblem is a quadratic program, with a quadratic model of the objective subject to a linearization of the constraints
* in SLQP, two subproblems are solved at each step: a linear program (LP) used to determine an active set, followed by an equality-constrained quadratic program (EQP) used to compute the total step

This decomposition makes SLQP suitable to large-scale optimization problems, for which efficient LP and EQP solvers are available, these problems being easier to scale than full-fledged quadratic programs.

It may be considered related to, but distinct from, quasi-Newton methods.

## Algorithm basics

Consider a nonlinear programming problem of the form:

    ${\begin{array}{rl}\min \limits _{x}&f(x)\\{\mbox{s.t.}}&b(x)\geq 0\\&c(x)=0.\end{array}}$

The Lagrangian for this problem is

    ${\mathcal {L}}(x,\lambda ,\sigma )=f(x)-\lambda ^{T}b(x)-\sigma ^{T}c(x),$

where $\lambda \geq 0$ and $\sigma$ are Lagrange multipliers.

### LP phase

In the LP phase of SLQP, the following linear program is solved:

    ${\begin{array}{rl}\min \limits _{d}&f(x_{k})+\nabla f(x_{k})^{T}d\\\mathrm {s.t.} &b(x_{k})+\nabla b(x_{k})^{T}d\geq 0\\&c(x_{k})+\nabla c(x_{k})^{T}d=0.\end{array}}$

Let ${\cal {A}}_{k}$ denote the active set at the optimum $d_{\text{LP}}^{*}$ of this problem, that is to say, the set of constraints that are equal to zero at $d_{\text{LP}}^{*}$. Denote by $b_{{\cal {A}}_{k}}$ and $c_{{\cal {A}}_{k}}$ the sub-vectors of $b$ and $c$ corresponding to elements of ${\cal {A}}_{k}$.

### EQP phase

In the EQP phase of SLQP, the search direction $d_{k}$ of the step is obtained by solving the following equality-constrained quadratic program:

    ${\begin{array}{rl}\min \limits _{d}&f(x_{k})+\nabla f(x_{k})^{T}d+{\tfrac {1}{2}}d^{T}\nabla _{xx}^{2}{\mathcal {L}}(x_{k},\lambda _{k},\sigma _{k})d\\\mathrm {s.t.} &b_{{\cal {A}}_{k}}(x_{k})+\nabla b_{{\cal {A}}_{k}}(x_{k})^{T}d=0\\&c_{{\cal {A}}_{k}}(x_{k})+\nabla c_{{\cal {A}}_{k}}(x_{k})^{T}d=0.\end{array}}$

Note that the term $f(x_{k})$ in the objective functions above may be left out for the minimization problems, since it is constant.

