### A Formula for The Selmer Group of a Rational Three-Isogeny, by Matt DeLong

A formula is given for the dimension of the Selmer group of the rational
three-isogeny of elliptic curves of the form y^2=x^3+a(x-b)^2. The
formula is in terms of the three-ranks of the quadratic number fields
Q(\sqrt{a}) and Q(\sqrt{-3a}) and various aspects of the arithmetic of
these number fields. In addition a duality theorem is used to relate the
dimension of the Selmer group of the three-isogeny with the dimension of
the Selmer group of its dual isogeny.

Matt DeLong <mtdelong@tayloru.edu>