Maybe it is too late. A lattice has a finite orthogonal group if and only if it is definite or it is on the hyperbolic plane. This can be deduced from Satz (30.4) in Kneser's book Quadratische Formen (taking L = M there).
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I correct the statement to "....it is definite or it is on the hyperbolic plane..."
WKC
- 646
- 4
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WKC
- 646
- 4
- 9