The principle of symmetric criticality

It is frequently explicitly or implicitly assumed that if a variational principle is invariant under some symmetry group G, then to test whether a symmetric field configuration phi is an extremal, it suffices to check the vanishing of the first variation of the action corresponding to variations phi + delta phi that are also symmetric. We show by example that this is not valid in complete generality (and in certain cases its meaning may not even be clear) and on the other hand prove some theorems which validate its use under fairly general circumstances (in particular if G is a group of Riemannian isometries, or if it is compact, or with some restrictions if it is semi-simple). (orig.)