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A characterization of the Zn ⊕Z(δ) lattice and definite nonunimodular intersection forms
- American Journal of Mathematics
- Johns Hopkins University Press
- Volume 134, Number 4, August 2012
- pp. 891-913
- 10.1353/ajm.2012.0026
- Article
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We prove a generalization of Elkies' characterization of the ${\Bbb{Z}}^n$
lattice to nonunimodular definite
forms (and lattices). Combined with inequalities of Fr{\o}yshov and of Ozsv\'{a}th and Szab\'{o}, this
gives a simple test of whether a rational homology three-sphere may bound a definite four-manifold. As an
example we show that small positive surgeries on torus knots do not bound negative-definite four-manifolds.