English
If x ≡ y mod U and f is linear, then f(x) ≡ f(y) mod U.map f.
Русский
Если x ≡ y mod U и f линейно, тогда f(x) ≡ f(y) mod U.map f.
LaTeX
$$$$ x \\equiv y\\ [SMOD\\ U] \\Rightarrow f x \\equiv f y\\ [SMOD\\ (U.map\\ f)]. $$$$
Lean4
theorem map (hxy : x ≡ y [SMOD U]) (f : M →ₗ[R] N) : f x ≡ f y [SMOD U.map f] :=
(Submodule.Quotient.eq _).2 <| f.map_sub x y ▸ mem_map_of_mem <| (Submodule.Quotient.eq _).1 hxy