# Second order cone programming

Let us continue with our regression problem from the linear and quadratic programming tutorials.

The problem boiled down to solving the problem minimize \(\left\lVert Ax - y\right\rVert \) for some suitable norm. Let us select the 2-norm, but this time solve the extension where we minimize the worst-case cost, under the assumption that the matrix \(A\) is uncertain, \(A=A+d\), where \(\left\lVert d\right\rVert_2 \leq 1 \) . This can be shown to be equivalent to minimizing \(\left\lVert Ax - y\right\rVert_2 \) + \(\left\lVert x\right\rVert_2 \).

This problem can easily be solved using YALMIP. Begin by defining the data.

```
x = [1 2 3 4 5 6]';
t = (0:0.02:2*pi)';
A = [sin(t) sin(2*t) sin(3*t) sin(4*t) sin(5*t) sin(6*t)];
e = (-4+8*rand(length(A),1));
y = A*x+e;
```

As a first approach, we will do the modelling by hand, by adding second-order cones using the low-level command cone.

```
xhat = sdpvar(6,1);
sdpvar u v
F = [cone(y-A*xhat,u), cone(xhat,v)];
solvesdp(F,u + v);
```

By using the automatic modelling support in the nonlinear operator framework, we can alternatively write it in the following epigraph form

```
xhat = sdpvar(6,1);
sdpvar u v
F = [norm(y-A*xhat,2) <= u, norm(xhat,2) <= v];
solvesdp(F,u + v);
```

Of course, we can write it in the most natural form (and YALMIP will automatically perform the epigraph reformulation and represent the model using second order cone operators)

```
solvesdp([],norm(y-A*xhat,2) + norm(xhat,2));
```

YALMIP will automatically model this as a second order cone problem, and solve it as such if a second-order cone programming solver is installed. If no second order cone programming solver is found, YALMIP will convert the model to a semidefinite program and solve it using any installed semidefinite programming solver

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