I am trying to get a concrete handle on the isomorphism $H^4(K(\pi\_2,2),U(1)) \simeq \{$quadratic forms $\pi\_2 \to U(1) \}$. This is explained in Eilenberg and Maclane's <http://www.jstor.org/stable/1969702> and its companion but I am having a hard time getting just what this 4-cocycle should assign to a 4-simplex in $K(\pi\_2,2)$. I am primarily interested in understanding the map from the right to the left.


I have a guess at something which may be close, which is there is a canonical closed 2-form on $K(\pi\_2,2)$ valued in $\pi\_2$. Using the associated bilinear form of the given quadratic form, I can wedge this form with itself to obtain a closed 4-form valued in $U(1)$. I worry that instead of the ordinary square, I need to be doing some factoring, perhaps using the Pontryagin square instead.


Any help, especially with some intuition, would be much appreciated.



On quadratic forms, Pontryagin Squares, and H^4(K(pi,2),U(1))
To a quadratic form $q: \pi\_2 \to U(1)$, we get a corresponding Pontryagin square operation $H^2(-; \pi\_2) \to H^4(-; U(1))$, and such cohomology operations are given by elements of $H^4(K(\pi\_2, 2); U(1))$. Unfortunately, I don't know if it's possible to get an explicit cochain-level description of the Pontryagin square operator just from the quadratic form. In Proposition 7.3 of my paper [*Extensions of groups by braided 2-groups*](http://arxiv.org/pdf/1106.0772.pdf), I write down a (group) cochain-level description of the Pontryagin square corresponding to an abelian 3-cocycle, but I don't know if there's a way in general to get an abelian 3-cocycle from a quadratic form. One good reference to look at is Baues's *Combinatorial Homotopy and 4-Dimensional Complexes*.


One intuitive way to think of the appearance of the Pontryagin square is that we are describing braided 2-groups with homotopy groups $\pi\_2$ and $U(1)$, and the Pontryagin square operation $H^2(G; \pi\_2) \to H^4(G; U(1))$ on a group $G$ is the obstruction to lifting an extension of $G$ by $\pi\_2$ to an extension of $G$ by the entire braided 2-group (in the sense I describe in my paper).


EDIT: I should mention that you can avoid referring to the Pontryagin square directly. Per Baues, $H\_4(K(\pi\_2, 2); \mathbb{Z}) = \Gamma(\pi\_2)$, the receptor of the universal quadratic map from $\pi\_2$, so by universal coefficients, $H^4(K(\pi\_2, 2); U(1)) = \operatorname{Hom}\_{\mathbb{Z}}(\Gamma(\pi\_2), U(1))$, which is the same as quadratic forms on $\pi\_2$ valued in $U(1)$.


