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Gruber P. Convex and Discrete Geometry

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

index, say i, <strong>and</strong> let c = ib1, where b1 ∈ L is primitive. Now the following will be<br />

shown:<br />

(5) Let m ∈ M \{o} <strong>and</strong> let j be the index of m. Then i| j.<br />

If m is linearly dependent on c, Proposition (5) is easy to see. Assume now that c<br />

<strong>and</strong> m are linearly independent. The 2-dimensional lattice M ′ = lin{c, m}∩M is a<br />

sub-lattice of the 2-dimensional lattice L ′ = lin{c, m} ∩L. Since b1 is a primitive<br />

point of L, <strong>and</strong> thus of L ′ , Corollary 21.1 shows that there is a basis of the form<br />

{b1, b2} of L ′ .Letm = j (ub1 + vb2), where u,v are relatively prime integers. Note<br />

that c = ib1 <strong>and</strong> choose h ∈ Z, such that<br />

n = m − hc = ( ju − ih)b1 + ( jv)b2 = k(wb1 + zb2), where<br />

0 ≤ ju − ih = kw

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