Suppose πi i = 1,2 are
principal K-bundles which are Cr-isomorphic in the sense that there exists a
K-equivariant Cr-diffeomorphism f : 𝒫1→𝒫2. If h belongs to the gauge group H2
of 𝒫2 then h∘f lies in H1 and we have a group isomorphism H2→ H1 which is C∞.
It is the purpose of this paper to investigate the converse in the case where K is a
simple Lie group. (If K is abelian the gauge group of every K bundle over X is
Cr(X,K) so there is no hope of a converse. However for simple groups the situation
is much better).