Lemma 10.4. Assuming that:

$E ∕ ℚ$ is given by a Weierstrass equation with $a_{1}, \dots, a_{6} \in ℤ$
$0 \neq T = (x, y) \in E {(ℚ)}_{tors}$

Then

(i) $4 x, 8 y \in ℤ$
(ii) If $2 | a_{1}$ or $2 T \neq 0$ then $x, y \in ℤ$