Variationnal study of ferromagnetism in the t1-t2 Hubbard chain
  A one-dimensional Hubbard model with nearest and (negative) next-nearest neighbour hopping is studied variationally. This allows to exclude saturated ferromagnetism for $U < U_c$. The variational boundary $U_c (n)$ has a minimum at a ``critical density'' $n_c$ and diverges for $n \rightarrow 1$. 
S. Daul, P. Pieri, M. Dzierzawa, D. Baeriswyl and P. Fazakas