group cohomology, nonabelian group cohomology, Lie group cohomology
cohomology with constant coefficients / with a local system of coefficients
differential cohomology
(see also Chern-Weil theory, parameterized homotopy theory)
A cubical structure on a complex line bundle over an abelian group is a certain trivialization of a certain induced line bundle on the 3-fold Cartesian product (“cube”) of the group which is constructed in a kind of cubical generalization of the polarization identity formula for quadratic forms.
Over formal groups associated with complex oriented cohomology theories cubical structures encode orientation in generalized cohomology.
Given a circle group-principal bundle/complex line bundle $\mathcal{L}$ on an abelian group $A$, write $\Theta(\mathcal{L})$ for the line bundle on $G^3$ which is given by the formula
A cubical structure on $\mathcal{L}$ is a trivializing section $s$ of $\Theta(\mathcal{L})$ such that
$s(0,0,0) = 1$
$s(x_{\sigma(1)}, x_{\sigma(2)}, x_{\sigma(3)}) = s(x_1, x_2, x_3)$
$s(w+x,y,z) s(w,x,z) = s(w,x + y, z) s(x,y,z)$
for all elements of $A$ as indicated, and for all permutations $\sigma$ of three elements. Here the equalities are equalities of section after applying the canonical isomorphisms of complex lines on both sides.
(Breen 83, Hopkins 94, section 4, Ando-Hopkins-Strickland 01, def. 2.40)
The canonical isomorphsms hidden in def. are:
$\mathcal{L}_0^{\otimes 3} \otimes (\mathcal{L}_0^{-1})^{\otimes 3} \to 1$ the canonical map exhibiting $\mathcal{L}_0^{-1}$ as the inverse (dual object) of $\mathcal{L}_0$:
etc.
There is the following further refinement.
In the situation of def. , if the line bundle $\mathcal{L}$ is equipped with a natural “symmetry”
then a $\Sigma$-structure on $\mathcal{L}$ is a cubical structure, def. , such that in addition
For $E$ a multiplicative weakly periodic complex orientable cohomology theory then $Spec E^0(B U\langle 6\rangle)$ is naturally equivalent to the space of cubical structures on the trivial line bundle over the formal group of $E$.
In particular, homotopy classes of maps of E-infinity ring spectra $MU\langle 6\rangle \to E$ from the Thom spectrum to $E$, and hence universal $MU\langle 6\rangle$-orientations (see there) of $E$ are in natural bijection with these cubical structures.
(Hopkins 94, theorem 6.1, 6.2, Ando-Hopkins-Strickland 01, corollary 2.50)
This way for instance the string orientation of tmf has been constructed. See there for more on this.
The 11-dimensiona Chern-Simons action functional in 11-dimensional supergravity gives a line bundle $L$ on the space of supergravity C-fields whose $\Theta^3(L)$ is the transgression of the cup product in ordinary differential cohomology of three factors. It seems that each trivialization of the class of the supergravity C-field induces a “cubical” trivialization of $\Theta^3(L)$ as above, and hence a cubical structure on $L$. See at cubical structure in M-theory for more on this.
An early reference discussing the relation with theta functions is
In relation to orientation in generalized cohomology cubical structures have been prominently discussed in
Michael Hopkins, Topological modular forms, the Witten genus, and the theorem of the cube, Proceedings of the International Congress of Mathematicians, Vol. 1, 2 (Zürich, 1994) (Basel), Birkhäuser, 1995, 554–565. MR 97i:11043 (pdf)
Matthew Ando, Michael Hopkins, Neil Strickland, Elliptic spectra, the Witten genus and the theorem of the cube, Invent. Math. 146 (2001) 595–687 MR1869850 (pdf)
Last revised on January 27, 2016 at 16:57:52. See the history of this page for a list of all contributions to it.