English
If s and t are balanced, then their sum s+t is balanced.
Русский
Если множества s и t сбалансированы, то их сумма s+t сбалансирована.
LaTeX
$$$\text{Balanced}_{\mathfrak{k}}(s) \wedge \text{Balanced}_{\mathfrak{k}}(t) \Rightarrow \text{Balanced}_{\mathfrak{k}}(s+t)$$$
Lean4
theorem add (hs : Balanced 𝕜 s) (ht : Balanced 𝕜 t) : Balanced 𝕜 (s + t) := fun _a ha =>
(smul_add _ _ _).subset.trans <| add_subset_add (hs _ ha) <| ht _ ha