English
If s and t are countable, then image2 f s t is countable.
Русский
Если s и t счётны, то image2 f s t счётно.
LaTeX
$$$\\forall s:\\ Set\\alpha, \\forall t:\\ Set\\beta, (s.Countable) \\land (t.Countable) \\Rightarrow (image2 f s t).Countable$$$
Lean4
theorem image2 {s : Set α} {t : Set β} (hs : s.Countable) (ht : t.Countable) (f : α → β → γ) :
(image2 f s t).Countable := by
rw [← image_prod]
exact (hs.prod ht).image _