We study a notion weakening the Einstein condition on a left invariant Riemannian metric g on a nilpotent Lie group N . We consider those metrics satisfying Ric g = cI + D for some c ∈ R and some derivation D of the Lie algebra n of N, where Ric g denotes the Ricci operator of (N, g). This condition is equivalent to the metric g to be a Ricci soliton. We prove that a Ricci soliton left invariant metric on N is unique up to isometry and scaling. The following characterization is also given: (N, g) is a Ricci soliton if and only if (N, g) admits a metric standard solvable extension whose corresponding standard solvmanifold (S,g) is Einstein. This gives several families of new examples of Ricci solitons. By a variational approach, we furthermore show that the Ricci soliton homogeneous nilmanifolds (N, g) are precisely the critical points of a natural functional defined on a vector space which contains all the homogeneous nilmanifolds of a given dimension as a real algebraic set. (2000): 53C30, 53C21, 53C25, 22E25.
Mathematics Subject Classification