It is shown that the first order (Palatini) variational principle for a generic nonlinear metric-affine Lagrangian depending on the (symmetrized) Ricci square invariant leads to an almost-product Einstein structure or to an almost-complex anti-Hermitian Einstein structure on a manifold. It is proved that a real anti-Hermitian metric on a complex manifold satisfies the Kähler condition on the sa...

Given a smooth family of unparameterized curves such that through every point in direction there passes exactly one curve, does exist Lagrangian with extremals being precisely this family? It is known dimension 2 the answer positive. In 3, it follows from work Douglas is, general, negative. We generalise result to all higher dimensions and show actually negative for almost curves, also as path ...

This paper develops a model of endogenous product selection by firms. The theory is motivated by new evidence we present on the importance of product switching by U.S. manufacturers. Two-thirds of continuing firms change their product mix every five years, and product switches involve more than 40% of firm output and almost half of existing products. The theoretical model incorporates heterogen...