Whitney immersion theorem

In differential topology, the Whitney immersion theorem (named after Hassler Whitney) states that for , any smooth -dimensional manifold (required also to be Hausdorff and second-countable) has a one-to-one immersion in Euclidean -space, and a (not necessarily one-to-one) immersion in -space. Similarly, every smooth -dimensional manifold can be immersed in the -dimensional sphere (this removes the constraint). The weak version, for , is due to transversality (general position, dimension counting): two m-dimensional manifolds in intersect generically in a 0-dimensional space. William S. Massey went on to prove that every n-dimensional manifold is cobordant to a manifold that immerses in where is the number of 1's that appear in the binary expansion of . In the same paper, Massey proved that for every n there is manifold (which happens to be a product of real projective spaces) that does not immerse in . The conjecture that every n-manifold immerses in became known as the immersion conjecture. This conjecture was eventually solved in the affirmative by .