# Cohomotopy group

(Redirected from Cohomotopy groups)

In mathematics, particularly algebraic topology, cohomotopy sets are particular contravariant functors from the category of pointed topological spaces and point-preserving continuous maps to the category of sets and functions. They are dual to the homotopy groups, but less studied.

The p-th cohomotopy set of a pointed topological space X is defined by

π p(X) = [X,S p]

the set of pointed homotopy classes of continuous mappings from X to the p-sphere S p. For p=1 this set has an abelian group structure, and, provided X is a CW-complex, is isomorphic to the first cohomology group H1(X), since S1 is a K(Z,1). In fact, it is a theorem of Hopf that if X is a CW-complex of dimension at most n, then [X,S p] is in bijection with the p-th cohomology group H p(X).

The set also has a group structure if X is a suspension ${\displaystyle \Sigma Y}$, such as a sphere Sq for q${\displaystyle \geq }$1.

If X is not a CW-complex, H 1(X) might not be isomorphic to [X,S 1]. A counterexample is given by the Warsaw circle, whose first cohomology group vanishes, but admits a map to S1 which is not homotopic to a constant map [1]

## Properties

Some basic facts about cohomotopy sets, some more obvious than others:

• π p(S q) = π q(S p) for all p,q.
• For q = p + 1 or p + 2 ≥ 4, π p(S q) = Z2. (To prove this result, Pontrjagin developed the concept of framed cobordisms.)
• If f,g: XS p has ||f(x) - g(x)|| < 2 for all x, [f] = [g], and the homotopy is smooth if f and g are.
• For X a compact smooth manifold, π p(X) is isomorphic to the set of homotopy classes of smooth maps XS p; in this case, every continuous map can be uniformly approximated by a smooth map and any homotopic smooth maps will be smoothly homotopic.
• If X is an m-manifold, π p(X) = 0 for p > m.
• If X is an m-manifold with boundary, π p(X,∂X) is canonically in bijection with the set of cobordism classes of codimension-p framed submanifolds of the interior X-∂X.
• The stable cohomotopy group of X is the colimit
${\displaystyle \pi _{s}^{p}(X)=\varinjlim _{k}{[\Sigma ^{k}X,S^{p+k}]}}$
which is an abelian group.

## References

1. ^ Polish Circle Retrieved July 17, 2014