For an elliptic selfadjoint operatoracting on a fiber bundle over a Riemannian manifold, where u µν , v µ , w are N × N -matrices, we develop a method to compute the heat-trace coefficients a r which allows to get them by a pure computational machinery. It is exemplified in dimension 4 by the value of a 1 written both in terms of u µν = g µν u, v µ , w or diffeomorphic and gauge invariants. We also answer the question: when is it possible to get explicit formulae for a r ?