A principal-symbol obstruction in metric-affine Gauss--Bonnet gravity

Abstract

The four-dimensional Riemannian Gauss--Bonnet combination admits more than one continuation to a geometry with an independent affine connection. Two natural candidates, the metric-contracted representative and the double-epsilon representative, agree in their Levi-Civita limit, and we ask whether they also give the independent connection the same leading differential structure. In a metric-affine model with a scalar coupling $f(ϕ)$ to the Gauss--Bonnet term, we compare their quadratic two-derivative connection principal forms about the Levi-Civita locus. For the metric-contracted continuation, the principal form, evaluated on a twenty-four-dimensional hook-free torsion-free witness subspace at a timelike principal covector, has rank $18$ and nullity $6$ and is indefinite; the full torsion-free form at the same covector is therefore indefinite with rank at least $18$. Locally where $f(ϕ)\neq0$, the two-derivative principal symbol of the linearized connection equations is nonzero, which obstructs purely algebraic local elimination of the connection in the activated directions. For the double-epsilon continuation, the corresponding quadratic two-derivative form vanishes identically for an arbitrary perturbation of the connection about that locus, so its principal symbol vanishes with it, and that particular obstruction is absent, though its absence does not by itself establish algebraic eliminability. Thus the leading differential structure of the independent connection depends on the affine continuation of the Riemannian Gauss--Bonnet combination.