English
If a and b represent the same quotient class modulo H, then they represent the same quotient class modulo G.
Русский
Если a и b эквивалентны по отношению к подгруппе H в фактор-множестве, то они эквивалентны по отношению к G.
LaTeX
$$$(\llbracket a \rrbracket : \text{orbitRel.Quotient } H\; \alpha) = (\llbracket b \rrbracket) \\Rightarrow (\llbracket a \rrbracket : \text{orbitRel.Quotient } G\; \alpha) = (\llbracket b \rrbracket)$$$
Lean4
@[to_additive]
theorem orbit_injective : Injective (orbitRel.Quotient.orbit : orbitRel.Quotient G α → Set α) :=
by
intro x y h
simp_rw [orbitRel.Quotient.orbit_eq_orbit_out _ Quotient.out_eq', orbit_eq_iff, ← orbitRel_apply] at h
simpa [← Quotient.eq''] using h