The purpose of this paper is to give a "down--to--earth" proof of the
well--known fact that a randomly chosen elliptic curve over the rationals is
most likely to have trivial torsion.
Let f be a weight two newform for Gamma_1(N) without complex multiplication.
In this article we study the conductor of the absolutely simple factors B of
the variety A_f over certain number fields L. The strategy we follow is to
compute the restriction of scalars Res_{L/\Q}(B), and then to apply Milne's
formula for the conductor of the restriction of scalars. In this way we obtain
an expression for the local exponents of the conductor N_L(B). Under some
hypothesis it is possible to give global formulas relating this conductor with
N.
We give several criteria to show over which quadratic number fields
$\bQ(\sqrt{D})$ there should exists a non-constant arithmetic progressions of
five squares. This is done by translating the problem to determining when some
genus five curves C_D defined over Q have rational points, and then using a
Mordell-Weil sieve argument among others.
We study the problem of the existence of arithmetic progressions of three
cubes over quadratic number fields Q(sqrt(D)), where D is a squarefree integer.
For this purpose, we give a characterization in terms of Q(sqrt(D))-rational
points on the elliptic curve E:y^2=x^3-27. We compute the torsion subgroup of
the Mordell-Weil group of this elliptic curve over Q(sqrt(D)) and we give
partial answers to the finiteness of the free part of E(Q(sqrt(D))). This last
task will be translated to compute if the rank of the quadratic D-twist of the
modular curve X_0(36) is zero or not.
We study the problem of the existence of arithmetic progressions of three
cubes over quadratic number fields Q(sqrt(D)), where D is a squarefree integer.
For this purpose, we give a characterization in terms of Q(sqrt(D))-rational
points on the elliptic curve E:y^2=x^3-27. We compute the torsion subgroup of
the Mordell-Weil group of this elliptic curve over Q(sqrt(D)) and we give
partial answers to the finiteness of the free part of E(Q(sqrt(D))). This last
task will be translated to compute if the rank of the quadratic D-twist of the
modular curve X_0(36) is zero or not.