Hilbert basis (linear programming)

The Hilbert basis of a convex cone C is a minimal set of integer vectors such that every integer vector in C is a conical combination of the vectors in the Hilbert basis with integer coefficients.

Given a lattice ${\displaystyle L\subset {\mathbb{\mathbb{Z}}}^d}$ and a convex polyhedral cone with generators ${\displaystyle a_1,\dots ,a_n\in {\mathbb{\mathbb{Z}}}^d}$

${\displaystyle C=\{{\lambda }_1{a}_1+\dots +{\lambda }_n{a}_n\mid {\lambda }_1,\dots ,{\lambda }_n\ge 0,{\lambda }_1,\dots ,{\lambda }_n\in \mathbb{ℝ}\}\subset {\mathbb{ℝ}}^d,}$

we consider the monoid ${\displaystyle C\cap L}$. By Gordan's lemma, this monoid is finitely generated, i.e., there exists a finite set of lattice points ${\displaystyle \{x_1,\dots ,x_m\}\subset C\cap L}$ such that every lattice point ${\displaystyle x\in C\cap L}$ is an integer conical combination of these points:

${\displaystyle x={\lambda }_1{x}_1+\dots +{\lambda }_m{x}_m,{\lambda }_1,\dots ,{\lambda }_m\in \mathbb{\mathbb{Z}},{\lambda }_1,\dots ,{\lambda }_m\ge 0.}$

The cone C is called pointed if ${\displaystyle x,-x\in C}$ implies ${\displaystyle x=0}$. In this case there exists a unique minimal generating set of the monoid ${\displaystyle C\cap L}$—the Hilbert basis of C. It is given by the set of irreducible lattice points: An element ${\displaystyle x\in C\cap L}$ is called irreducible if it can not be written as the sum of two non-zero elements, i.e., ${\displaystyle x=y+z}$ implies ${\displaystyle y=0}$ or ${\displaystyle z=0}$.

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