Talk:Reduction of the structure group

Seems to me that there's a fundamental problem here. If we're not talking about principle G-bundles, but just about bundles with a given structure group G, then I don't see why there would be any action of G (from the left or from the right). As a simple example, consider the tangent space to the two dimensional sphere. Let A be some invertible matrix. How does it act on this space? The only exception are scalar matrices, since they are in the center. In general, the larger G is, the smaller the group of automorphisms of the fiber which can act on the whole space - these are exactly those automorphisms which respect the action of G on the fiber.

I think the entire article should be rewritten carefuly, distinguishing between principle and non-principle bundles. Of course I could have a mistake in my reasoning (I'm still learning this stuff...) Either way I will appreciate your comments on this...

Amitushtush (talk) 00:10, 6 August 2009 (UTC)

Even for a general G-bundle, there is a canonical action of G on the whole space. Define it in a local trivialization by $g\mapsto [(x,f)\mapsto (x,gf)]$. As you might expect, the transition maps are exactly what you need to make this well-defined.

Neocapitalist (talk) 18:45, 5 December 2010 (UTC)

The transition maps for a principal G-bundle are by left multiplication, and the action of G on the bundle can then be defined in a local trivialization by right multiplication (because left and right multiplication commute). For a general G-bundle, however, it seems to me that you need two commuting actions of G on the fiber to get an action of G on the space, which you don't have in general. I believe Amitushtush is right in that a general G-bundle does not have a G-action on it.

As for the example with the two-sphere: the matrix diag(1,2) would have to act on some way in the tangent space at the north pole, but it would have to act by a scalar since any other action would 'break' the rotational symmetry.

However, I believe most of the article holds for general G-bundles, since no action of G is required. A G-action is required for the last part of 'Definition' (the correspondence between reductions to H and sections of the quotient bundle).

63.252.64.200 (talk) 20:49, 2 December 2013 (UTC)