English
If s and t are inf-closed, then their product s × t is inf-closed.
Русский
Если s и t инф-замкнуты, то их произведение инф-замкнуто.
LaTeX
$$$ (\\text{InfClosed } s) \\rightarrow (\\text{InfClosed } t) \\rightarrow \\text{InfClosed}(s \\times t) $$$
Lean4
theorem prod {t : Set β} (hs : InfClosed s) (ht : InfClosed t) : InfClosed (s ×ˢ t) := fun _a ha _b hb ↦
⟨hs ha.1 hb.1, ht ha.2 hb.2⟩