English
If two matrices have the same action on all standard basis vectors (Pi.single i 1), then the matrices are equal.
Русский
Если две матрицы действуют одинаково на все базисные векторы, то они равны.
LaTeX
$$If ∀ i, M ·ᵥ e_i = N ·ᵥ e_i then M = N, where e_i = Pi.single i 1.$$
Lean4
theorem ext_of_mulVec_single [DecidableEq n] [Fintype n] {M N : Matrix m n α}
(h : ∀ i, M *ᵥ Pi.single i 1 = N *ᵥ Pi.single i 1) : M = N :=
by
ext i j
simp_rw [mulVec_single_one] at h
exact congrFun (h j) i