  We classify complete gradient Ricci solitons satisfying a fourth-order
vanishing condition on the Weyl tensor, improving previously known results.
More precisely, we show that any $n$-dimensional ($n\geq 4$) gradient shrinking
Ricci soliton with fourth order divergence-free Weyl tensor is either Einstein,
or a finite quotient of $N^{n-k}\times \mathbb{R}^k$, $(k > 0)$, the product of
a Einstein manifold $N^{n-k}$ with the Gaussian shrinking soliton
$\mathbb{R}^k$. The technique applies also to the steady and expanding cases in
all dimensions. In particular, we prove that a three dimensional gradient
steady soliton with third order divergence-free Cotton tensor, i.e. with
vanishing double divergence of the Bach tensor, is either flat or isometric to
the Bryant soliton.

Differential Geometry