# Hermite problem

The problem about the homogeneous arithmetical minima of positive $n$- ary quadratic forms with real coefficients. It is equivalent to the problem of the densest lattice packing of $n$- dimensional balls of equal radius (see Geometry of numbers).

Let $f = f ( x)$, $x \in \mathbf R ^ {n}$, be a positive quadratic form over $\mathbf R$ of determinant $d = d ( f ) = \mathop{\rm det} f \neq 0$; and let

$$m ( f ) = \inf _ {\begin{array}{c} x \in \mathbf Z ^ {n} \\ x \neq 0 \end{array} } f ( x) = \min _ {\begin{array}{c} x \in \mathbf Z ^ {n} \\ {x \neq 0 } \end{array} } f ( x)$$

be its homogeneous arithmetical minimum. The quantity

$$\gamma _ {n} = \sup \frac{m ( f ) }{\{ d ( f ) \} ^ {1/n} } = \max \ \frac{m ( f ) }{\{ d ( f ) \} ^ {1/n} } ,$$

where the supermum or maximum is over all positive quadratic forms $f$, is called the Hermite constant; $\gamma _ {n} = \{ \gamma ( F _ {n} ) \} ^ {2}$, where $F _ {n} ( x) = ( x _ {1} ^ {2} + \dots + x _ {n} ^ {2} ) ^ {1/2}$ is the radial function corresponding to a ball.

Originally, one understood by the Hermite problem the task of finding or estimating $\gamma _ {n}$( from above and below). The exact values of $\gamma _ {n}$ are known only for $n \leq 8$( see ). For estimates of $\gamma _ {n}$, see  or .

Subsequently, the term Hermite problem was used for the search for local maxima (boundary or extremal) of $m ( f ) / \{ d ( f ) \} ^ {1/n}$ in the space of coefficients and their corresponding forms $f$. Algorithms are known for finding all classes of boundary forms. In particular, the Voronoi algorithm for perfect forms (see , , ).

The problem was posed by Ch. Hermite in 1850.

How to Cite This Entry:
Hermite problem. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Hermite_problem&oldid=47216
This article was adapted from an original article by A.V. Malyshev (originator), which appeared in Encyclopedia of Mathematics - ISBN 1402006098. See original article