A Remark on the Instability of the Bartnik-McKinnon Solutions

The aim of the present letter is to critically review the stability of the Bartnik-McKinnon solutions of the Einstein-Yang-Mills theory. The stability question was already studied by several authors, but there seems to be some confusion about the nature and the number of unstable modes. We suggest to distinguish two different kind of instabilities, which we call `gravitational' respectively `sphaleron' instabilities. We claim that the $n^{\rm th}$ Bartnik-McKinnon solution has exactly $2n$ unstable modes, $n$ of either type.