On the Laplacian coflow of invariant $G_2$-structures and its solitons

Abstract: In this work, we approach the Laplacian coflow of a coclosed $G_2$-structure $\varphi$ using the formulae for the irreducible $G_2$-decomposition of the Hodge Laplacian and the Lie derivative of the Hodge dual $4$-form of $\varphi$. In terms of this decomposition, we characterise the conditions for a vector field as an infinitesimal symmetry of a coclosed $G_2$-structure, as well as the soliton condition for the Laplacian coflow. More specifically, we provide an easier proof for the absence of compact shrinking solitons of the Laplacian coflow. Moreover, we revisit the Laplacian coflow of coclosed $G_2$-structures on almost Abelian Lie groups addressed by Fino-Bagaglini (2018). However, our approach is based on the bracket flow point of view. Notably, by showing that the norm of the Lie bracket is strictly decreasing, we prove that we have long-time existence for any coclosed Laplacian coflow solution.