Purely coclosed G_2 -structures on 2-step nilpotent Lie groups

We consider left-invariant (purely) coclosed G2-structures on 7-dimensional 2-step nilpotent Lie groups. According to the dimension of the commutator subgroup, we obtain various criteria characterizing the Riemannian metrics induced by left-invariant purely coclosed G2-structures. Then, we use them to determine the isomorphism classes of 2-step nilpotent Lie algebras admitting such type of structures. As an intermediate step, we show that every metric on a 2-step nilpotent Lie algebra admitting coclosed G2-structures is induced by one of them. Finally, we use our results to give the explicit description of the metrics induced by purely coclosed G2-structures on 2-step nilpotent Lie algebras with derived algebra of dimension at most two, up to automorphism.