For an abelian structure group A, principal A-bundles over a smooth manifold M can be described completely in terms of their holonomy functionals f:LMtoA.
Here, LM is the thin loop space whose elements are homotopy classes of loops, with the rank of the homotopies bounded by one. A map f:LMtoA is by definition smooth, if its pullback to the ordinary loop space is smooth (in the Fréchet sense).
Definition. A fusion map is a smooth map f:LMtoA such that for every triple (gamma1,gamma2,gamma3) of paths in M, with a common initial point and a common end point, and each path is smoothly composable with the inverse of each other, the map satisfies
f(overlinegamma2stargamma1)cdotf(overlinegamma3stargamma2)=f(overlinegamma3stargamma1).
Theorem.
The group of isomorphism classes of principal A-bundles with connection over M is isomorphic to the group of fusion maps.
The group of isomorphism classes of principal A-bundles with flat connection over M is isomorphic to the group of locally constant fusion maps.
The group of isomorphism classes of principal A-bundles over M is isomorphic to the group of homotopy classes of fusion maps (where the homotopies are smooth and go through fusion maps).
Parts 1 and 2 are a theorem of J. Barrett, proved in "Holonomy and Path Structures in General Relativity and Yand-Mills Theory". Part 3 is in my paper "Transgression to Loop Spaces and its Inverse I".
Corollary.
Two principal A-bundles with connection (no matter if flat or not) are isomorphic as bundles, if and only if their holonomy functionals are homotopic in the sense of 3.
The non-abelian case should be similiar. In fact, Barrett's result is valid for non-abelian groups.
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