English
If q has a root a, then multiplying it by any polynomial p preserves the root: IsRoot q a → IsRoot (p · q) a.
Русский
Если q имеет корень a, то умножение на любой полином p сохраняет корень: IsRoot q a → IsRoot (p · q) a.
LaTeX
$$$\\text{IsRoot}(p\\cdot q, a) \\leftarrow\\text{from } \\text{IsRoot}(q,a)$; more precisely, $\\text{IsRoot } q a \\Rightarrow \\text{IsRoot } (p q) a$.$$
Lean4
/-- `comp p`, regarded as a ring homomorphism from `R[X]` to itself. -/
def compRingHom : R[X] → R[X] →+* R[X] :=
eval₂RingHom C