Abstract
We prove a local-global principle for the problem of representations of quadratic forms by quadratic forms over ℤ, in codimension ≥5. The proof uses the ergodic theory of p-adic groups, together with a fairly general observation on the structure of orbits of an arithmetic group acting on integral points of a variety.
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Ellenberg, J., Venkatesh, A. Local-global principles for representations of quadratic forms. Invent. math. 171, 257–279 (2008). https://doi.org/10.1007/s00222-007-0077-7
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DOI: https://doi.org/10.1007/s00222-007-0077-7