Minkowski's theorem

In mathematics, Minkowski's theorem is the statement that any convex set in Rⁿ which is symmetric with respect to the origin and with volume greater than 2ⁿ d(L) contains a non-zero lattice point. The theorem was proved by Hermann Minkowski in 1889 and became the foundation of the branch of number theory called the geometry of numbers.

Suppose that L is a lattice of determinant d(L) in the n-dimensional real vector space Rⁿ and S is a convex subset of Rⁿ that is symmetric with respect to the origin, meaning that if x is in S then −x is also in S. Minkowski's theorem states that if the volume of S is strictly greater than 2ⁿ d(L), then S must contain at least one lattice point other than the origin.

The simplest example of a lattice is the set Zⁿ of all points with integer coefficients; its determinant is 1. For n = 2, the theorem claims that a convex figure in the plane symmetric about the origin and with area greater than 4 encloses at least one lattice point in addition to the origin. The area bound is sharp: if S is the interior of the square with vertices (±1, ±1) then S is symmetric and convex, has area 4, but the only lattice point it contains is the origin. This observation generalizes to every dimension n.

The following argument proves Minkowski's theorem for the special case of L=Z². It can be generalized to arbitrary lattices in arbitrary dimensions.

Consider the map ${\displaystyle f:S\to \mathbb {R} ^{2},(x,y)\mapsto (x\;{\bmod {\;}}2,y\;{\bmod {\;}}2)}$. Intuitively, this map cuts the plane into 2 by 2 squares, then stacks the squares on top of each other. Clearly ${\displaystyle f(S)}$ has area ≤ 4. Suppose f were injective, which means the pieces of S cut out by the squares stack up in a non-overlapping way. Since f is locally area-preserving, this non-overlapping property would make it area-preserving for all of S, so the area of f(S) would be the same as that of S, which is greater than 4. That is not the case, so f is not injective, and ${\displaystyle f(p_{1})=f(p_{2})}$ for some pair of points ${\displaystyle p_{1},p_{2}}$ in S. Moreover, we know from the definition of f that ${\displaystyle p_{2}=p_{1}+(2i,2j)}$ for some integers i and j, where i and j are not both zero.

...
Wikipedia