  Let N be a prime and let A be a quotient of J_0(N) over Q associated to a
newform such that the special L-value of A (at s=1) is non-zero. Suppose that
the algebraic part of the special L-value of A is divisible by an odd prime q
such that q does not divide the numerator of (N-1)/12. Then the Birch and
Swinnerton-Dyer conjecture predicts that the q-adic valuations of the algebraic
part of the special L-value of A and of the order of the Shafarevich-Tate group
are both positive even numbers. Under a certain mod q non-vanishing hypothesis
on special L-values of twists of A, we show that the q-adic valuations of the
algebraic part of the special L-value of A and of the Birch and Swinnerton-Dyer
conjectural order of the Shafarevich-Tate group of A are both positive even
numbers. We also give a formula for the algebraic part of the special L-value
of A over quadratic imaginary fields K in terms of the free abelian group on
isomorphism classes of supersingular elliptic curves in characteristic N
(equivalently, over conjugacy classes of maximal orders in the definite
quaternion algebra over Q ramified at N and infinity) which shows that this
algebraic part is a perfect square up to powers of the prime two and of primes
dividing the discriminant of K. Finally, for an optimal elliptic curve E, we
give a formula for the special L-value of the twist E_D of E by a negative
fundamental discriminant -D, which shows that this special L-value is an
integer up to a power of 2, under some hypotheses. In view of the second part
of the Birch and Swinnerton-Dyer conjecture, this leads us to the surprising
conjecture that the square of the order of the torsion subgroup of E_D divides
the product of the order of the Shafarevich-Tate group of E_D and the orders of
the arithmetic component groups of E_D, under certain mild hypotheses.
