A subset 𝑋⊆ℝ𝑁 is an 𝑛-dimensional real submanifold iff has charts that are 𝐶∞diffeomorphisms as subsets of real coördinate space,
i.e. for every 𝑥∈𝑋
there exists a neighbourhood 𝑈′ of 𝑥 in ℝ𝑁
and an open set 𝑉⊆ℝ𝑛
such that there exists a 𝐶∞ map 𝜑:𝑈′↠𝑉
with a 𝐶∞ right-inverse 𝜓=(𝜑↾𝑈′∩𝑋)−1.