Let K be a number field. We consider a local-global principle for elliptic
curves E/K that admit (or do not admit) a rational isogeny of prime degree n.
For suitable K (including K=Q), we prove that this principle holds when n = 1
mod 4, and for n < 7, but find a counterexample when n = 7 for an elliptic
curve with j-invariant 2268945/128. For K = Q we show that, up to isomorphism,
this is the only counterexample.Comment: 11 pages, minor edits, to appear in Journal de Th\'eorie des Nombres
de Bordeau