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Gruber P. Convex and Discrete Geometry

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406 <strong>Geometry</strong> of Numbers<br />

Here equality occurs precisely in case when there is an integer unimodular transformation<br />

of the variables, such that the linear forms assume the form β1v1,...,βdvd<br />

<strong>and</strong> α1 ≡ 1 2 β1 mod β1,...,αd ≡ 1 2 βd mod βd.<br />

The conjecture may also be expressed as follows:<br />

Conjecture 27.2. For any lattice L in E d the family of all translates of the set<br />

(1) |x1 ···xd| ≤ d(L)<br />

2 d<br />

by vectors of L covers E d . Precisely for the lattices L not of the form DZ d , where D<br />

is a d × d diagonal matrix, the set in (1) can be replaced by the set<br />

TheCasesd=2,...,6<br />

The conjecture is true for:<br />

|x1 ···xd| < d(L)<br />

.<br />

2d d = 2: Minkowski [737, 743], a multitude of further proofs<br />

d = 3: Remak [830], Davenport [243], who simplified Remak’s proof, Birch<br />

<strong>and</strong> Swinnerton-Dyer [117], Narzullaev [762]<br />

d = 4: Dyson [281], Skubenko [941], Bambah <strong>and</strong> Woods [60]<br />

d = 5: Skubenko [941]. Bambah <strong>and</strong> Woods [62] gave a proof along similar<br />

lines, clarifying Skubenko’s arguments<br />

d = 6: McMullen [704]<br />

Most of the proofs mentioned so far follow the line of proof of Remak-Davenport:<br />

For the proof of the conjecture (except for the equality case) it is sufficient to show<br />

the following two assertions:<br />

(i) For each lattice L in E d there is an ellipsoid E of the form:<br />

a1x 2 1 +···+ad x 2 d ≤ 1<br />

which contains d linearly independent points of L on its boundary but no<br />

point of L in its interior, except o.<br />

(ii) Let L be a lattice of determinant 1 <strong>and</strong> ρ B d a ball which contains d linearly<br />

independent points of L on its boundary but no point of L in its interior,<br />

except o. Then the following family of balls covers E d :<br />

� √ d<br />

2 Bd �<br />

+ l : l ∈ L .<br />

Let L be a lattice of determinant 1 <strong>and</strong> let E be the corresponding ellipsoid. After<br />

applying to L <strong>and</strong> E a suitable diagonal transformation of determinant 1, we may<br />

assume that E is a Euclidean ball as in (ii). Then the translates of the ball ( √ d/2)B d<br />

by the vectors of L cover E d . Since the inequality of the arithmetic <strong>and</strong> geometric<br />

mean shows that the ball ( √ d/2)B d is contained in the set |x1 ···xd| ≤1/2 d , the<br />

translates of this set by the vectors of L cover E d .

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