TITLE: Hopf invariant and the linking number
QUESTION [6 upvotes]: The Hopf invariant of a map $f:S^{2n-1}\to S^n$ can be defined in various ways,
in particular:
(1) as the linking number of the preimages of two points and
(2) using the cohomology ring of the space $S^n\cup_f D^{2n}$.
Where can I find a proof of equivalence of these two definitions?

REPLY [3 votes]: Historical reference is

Steenrod N. E. Cohomology invariants of mappings // Ann. Math. 1949.
  V. 50. P. 954—988.