# Standard form problem

MathOptInterface represents optimization problems in the standard form:

\begin{align} & \min_{x \in \mathbb{R}^n} & f_0(x) \\ & \;\;\text{s.t.} & f_i(x) & \in \mathcal{S}_i & i = 1 \ldots m \end{align}

where:

Tip

MOI defines some commonly used functions and sets, but the interface is extensible to other sets recognized by the solver.

## Functions

The function types implemented in MathOptInterface.jl are:

FunctionDescription
VariableIndex$x_j$, i.e., projection onto a single coordinate defined by a variable index $j$.
VectorOfVariablesThe projection onto multiple coordinates (i.e., extracting a sub-vector).
ScalarAffineFunction$a^T x + b$, where $a$ is a vector and $b$ scalar.
VectorAffineFunction$A x + b$, where $A$ is a matrix and $b$ is a vector.
ScalarQuadraticFunction$\frac{1}{2} x^T Q x + a^T x + b$, where $Q$ is a symmetric matrix, $a$ is a vector, and $b$ is a constant.
VectorQuadraticFunctionA vector of scalar-valued quadratic functions.

Extensions for nonlinear programming are present but not yet well documented.

## One-dimensional sets

The one-dimensional set types implemented in MathOptInterface.jl are:

SetDescription
LessThan(u)$(-\infty, u]$
GreaterThan(l)$[l, \infty)$
EqualTo(v)$\{v\}$
Interval(l, u)$[l, u]$
Integer()$\mathbb{Z}$
ZeroOne()$\{ 0, 1 \}$
Semicontinuous(l, u)$\{ 0\} \cup [l, u]$
Semiinteger(l, u)$\{ 0\} \cup \{l,l+1,\ldots,u-1,u\}$

## Vector cones

The vector-valued set types implemented in MathOptInterface.jl are:

SetDescription
Reals(d)$\mathbb{R}^{d}$
Zeros(d)$0^{d}$
Nonnegatives(d)$\{ x \in \mathbb{R}^{d} : x \ge 0 \}$
Nonpositives(d)$\{ x \in \mathbb{R}^{d} : x \le 0 \}$
SecondOrderCone(d)$\{ (t,x) \in \mathbb{R}^{d} : t \ge \lVert x \rVert_2 \}$
RotatedSecondOrderCone(d)$\{ (t,u,x) \in \mathbb{R}^{d} : 2tu \ge \lVert x \rVert_2^2, t \ge 0,u \ge 0 \}$
ExponentialCone()$\{ (x,y,z) \in \mathbb{R}^3 : y \exp (x/y) \le z, y > 0 \}$
DualExponentialCone()$\{ (u,v,w) \in \mathbb{R}^3 : -u \exp (v/u) \le \exp(1) w, u < 0 \}$
GeometricMeanCone(d)$\{ (t,x) \in \mathbb{R}^{1+n} : x \ge 0, t \le \sqrt[n]{x_1 x_2 \cdots x_n} \}$ where $n$ is $d - 1$
PowerCone(α)$\{ (x,y,z) \in \mathbb{R}^3 : x^{\alpha} y^{1-\alpha} \ge |z|, x \ge 0,y \ge 0 \}$
DualPowerCone(α)$\{ (u,v,w) \in \mathbb{R}^3 : \left(\frac{u}{\alpha}\right(^{\alpha}\left(\frac{v}{1-\alpha}\right)^{1-\alpha} \ge |w|, u,v \ge 0 \}$
NormOneCone(d)$\{ (t,x) \in \mathbb{R}^{d} : t \ge \sum_i \lvert x_i \rvert \}$
NormInfinityCone(d)$\{ (t,x) \in \mathbb{R}^{d} : t \ge \max_i \lvert x_i \rvert \}$
RelativeEntropyCone(d)$\{ (u, v, w) \in \mathbb{R}^{d} : u \ge \sum_i w_i \log (\frac{w_i}{v_i}), v_i \ge 0, w_i \ge 0 \}$
HyperRectangle(l, u)$\{x \in \bar{\mathbb{R}}^d: x_i \in [l_i, u_i] \forall i=1,\ldots,d\}$

## Matrix cones

The matrix-valued set types implemented in MathOptInterface.jl are:

SetDescription
RootDetConeTriangle(d)$\{ (t,X) \in \mathbb{R}^{1+d(1+d)/2} : t \le \det(X)^{1/d}, X \mbox{ is the upper triangle of a PSD matrix} \}$
RootDetConeSquare(d)$\{ (t,X) \in \mathbb{R}^{1+d^2} : t \le \det(X)^{1/d}, X \mbox{ is a PSD matrix} \}$
PositiveSemidefiniteConeTriangle(d)$\{ X \in \mathbb{R}^{d(d+1)/2} : X \mbox{ is the upper triangle of a PSD matrix} \}$
PositiveSemidefiniteConeSquare(d)$\{ X \in \mathbb{R}^{d^2} : X \mbox{ is a PSD matrix} \}$
LogDetConeTriangle(d)$\{ (t,u,X) \in \mathbb{R}^{2+d(1+d)/2} : t \le u\log(\det(X/u)), X \mbox{ is the upper triangle of a PSD matrix}, u > 0 \}$
LogDetConeSquare(d)$\{ (t,u,X) \in \mathbb{R}^{2+d^2} : t \le u \log(\det(X/u)), X \mbox{ is a PSD matrix}, u > 0 \}$
NormSpectralCone(r, c)$\{ (t, X) \in \mathbb{R}^{1 + r \times c} : t \ge \sigma_1(X), X \mbox{ is a } r\times c\mbox{ matrix} \}$
NormNuclearCone(r, c)$\{ (t, X) \in \mathbb{R}^{1 + r \times c} : t \ge \sum_i \sigma_i(X), X \mbox{ is a } r\times c\mbox{ matrix} \}$
HermitianPositiveSemidefiniteConeTriangle(d)The cone of Hermitian positive semidefinite matrices, with
side_dimension rows and columns.

Some of these cones can take two forms: XXXConeTriangle and XXXConeSquare.

In XXXConeTriangle sets, the matrix is assumed to be symmetric, and the elements are provided by a vector, in which the entries of the upper-right triangular part of the matrix are given column by column (or equivalently, the entries of the lower-left triangular part are given row by row).

In XXXConeSquare sets, the entries of the matrix are given column by column (or equivalently, row by row), and the matrix is constrained to be symmetric. As an example, given a 2-by-2 matrix of variables X and a one-dimensional variable t, we can specify a root-det constraint as [t, X11, X12, X22] ∈ RootDetConeTriangle or [t, X11, X12, X21, X22] ∈ RootDetConeSquare.

We provide both forms to enable flexibility for solvers who may natively support one or the other. Transformations between XXXConeTriangle and XXXConeSquare are handled by bridges, which removes the chance of conversion mistakes by users or solver developers.

## Multi-dimensional sets with combinatorial structure

Other sets are vector-valued, with a particular combinatorial structure. Read their docstrings for more information on how to interpret them.

SetDescription
SOS1A Special Ordered Set (SOS) of Type I
SOS2A Special Ordered Set (SOS) of Type II
IndicatorA set to specify an indicator constraint
ComplementsA set to specify a mixed complementarity constraint
AllDifferentThe all_different global constraint
BinPackingThe bin_packing global constraint
CircuitThe circuit global constraint
CountAtLeastThe at_least global constraint
CountBelongsThe nvalue global constraint
CountDistinctThe distinct global constraint
CountGreaterThanThe count_gt global constraint
CumulativeThe cumulative global constraint
PathThe path global constraint
TableThe table global constraint