# Noncommutative variables

**Adapted from**: Examples 2.11 and 2.2 of [BKP16]

[BKP16] Sabine Burgdorf, Igor Klep, and Janez Povh. *Optimization of polynomials in non-commuting variables*. Berlin: Springer, 2016.

## Example 2.11

We consider the Example 2.11 of [BKP16] in which the polynomial with noncommutative variables $(x * y + x^2)^2 = x^4 + x^3y + xyx^2 + xyxy$ is tested to be sum-of-squares.

```
using DynamicPolynomials
@ncpolyvar x y
p = (x * y + x^2)^2
```

\[ xyxy + xyx^{2} + x^{3}y + x^{4} \]

We first need to pick an SDP solver, see here for a list of the available choices.

```
using SumOfSquares
import CSDP
optimizer_constructor = optimizer_with_attributes(CSDP.Optimizer, MOI.Silent() => true)
model = Model(optimizer_constructor)
con_ref = @constraint(model, p in SOSCone())
optimize!(model)
```

We see that both the monomials `xy`

and `yx`

are considered separately, this is a difference with the commutative version.

`certificate_basis(con_ref)`

`MonomialBasis([xy, x²])`

We see that the solution correctly uses the monomial `xy`

instead of `yx`

. We also identify that only the monomials `x^2`

and `xy`

would be needed. This would be dectected by the Newton chip method of [Section 2.3, BKP16].

`gram_matrix(con_ref).Q`

```
2×2 SymMatrix{Float64}:
1.0 1.0
1.0 1.0
```

When asking for the SOS decomposition, the numerically small entries makes the solution less readable.

`sos_decomposition(con_ref)`

`(-1.0000000000000002*x*y - x^2)^2 + (-6.265166515512128e-9*x*y + 6.265166515512127e-9*x^2)^2`

They are however easily discarded by using a nonzero tolerance:

`sos_decomposition(con_ref, 1e-6)`

`(-1.0000000000000002*x*y - x^2)^2`

## Example 2.2

We consider now the Example 2.2 of [BKP16] in which the polynomial with noncommutative variables $(x + x^{10}y^{20}x^{10})^2$ is tested to be sum-of-squares.

```
using DynamicPolynomials
@ncpolyvar x y
n = 10
p = (x + x^n * y^(2n) * x^n)^2
using SumOfSquares
model = Model(optimizer_constructor)
con_ref = @constraint(model, p in SOSCone())
optimize!(model)
```

Only two monomials were considered for the basis of the gram matrix thanks to the Augmented Newton chip method detailed in [Section 2.4, BKP16].

```
certificate_basis(con_ref)
gram_matrix(con_ref).Q
sos_decomposition(con_ref, 1e-6)
```

`(-1.0000000000000002*x - x^10*y^20*x^10)^2`

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