We are interested in finding for which positive integers D we have rational solutions for the equation $$x^3+y^3=D.$$
x
3
+
y
3
=
D
.
The aim of this paper is to compute the value of the L-function $$L(E_D, 1)$$
L
(
E
D
,
1
)
for the elliptic curves $$E_D: x^3+y^3=D$$
E
D
:
x
3
+
y
3
=
D
. For the case of p prime $$p\equiv 1\mod 9$$
p
≡
1
mod
9
, two formulas have been computed by Rodriguez-Villegas and Zagier. We have computed formulas that relate $$L(E_D, 1)$$
L
(
E
D
,
1
)
to the square of a trace of a modular function at a CM point. This offers a criterion for when the integer D is the sum of two rational cubes. Furthermore, when $$L(E_D, 1)$$
L
(
E
D
,
1
)
is nonzero we get a formula for the number of elements in the Tate–Shafarevich group and we show that this number is a square when D is a norm in $${\mathbb {Q}}[\sqrt{-3}]$$
Q
[
-
3
]
.