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May 3, 2020 at 8:16 comment added Pat @VesselinDimitrov Thank you for this. Can you make this into an answer?
Apr 19, 2020 at 6:54 comment added Vesselin Dimitrov Yes, there is are infinitely many such special parameters $P$ at which $\sigma(P)$ becomes torsion in its fibre. Moreover, all large enough orders are realizable for the torsion point. This comes by a version of the implicit function theorem. When the data is over the algebraic numbers, one further knows that these special parameters $P$ are a set of bounded height (so they are rather sparse). See Masser and Zannier's paper Torsion anomalous points and families of elliptic curves and, for a generalization, Prop. 3.1 in Habegger's paper Special points on fibered powers of elliptic surfaces.
Apr 18, 2020 at 22:04 history asked Pat CC BY-SA 4.0