Past exam of the mathematics course of the University of Cambridge 2012 iii Paper 27 3 iii Solution Created 2026-10-03 Updated 2026-10-07
Direct substitution gives for both points. The displayed Weierstrass equation of an elliptic curve is minimal at every prime by the permitted assumption.
For , neither nor is divisible by three or five. ThereforeThe formal kernel of a minimal Weierstrass equation description gives . Similarly, neither nor is divisible by three or seven, so , with the same respective coordinate valuations .
If a point had finite order of a group element with , the point would be killed by . By prime-to-residue-characteristic multiplication on a formal group, multiplication by is injective on this kernel, so . Its order is therefore a power of . Applying this at two distinct primes is the two-prime formal-kernel test for nontorsion: the order of would be both a power of three and of five, and the order of both a power of three and of seven. Each would have to be the identity, although both are affine points. Both and have infinite order.