Isotropical Linear Spaces and Valuated Delta-Matroids

The spinor variety is cut out by the quadratic Wick relations among the principal Pfaffians of an n x n skew-symmetric matrix. Its points correspond to n-dimensional isotropic subspaces of a 2n-dimensional vector space. In this paper we tropicalize this picture, and we develop a combinatorial theory of tropical Wick vectors and tropical linear spaces that are tropically isotropic. We characterize tropical Wick vectors in terms of subdivisions of Delta-matroid polytopes, and we examine to what extent the Wick relations form a tropical basis. Our theory generalizes several results for tropical linear spaces and valuated matroids to the class of Coxeter matroids of type D

On Equivalence of M♮^\natural-concavity of a Set Function and Submodularity of Its Conjugate

A fundamental theorem in discrete convex analysis states that a set function is M$^\natural$-concave if and only if its conjugate function is submodular. This paper gives a new proof to this fact

Zero Nominal Interest Rates, Unemployment, Excess Reserves and Deflation in a Liquidity Trap

We present a dynamic and monetary model that consistently explains such various phenomena as unemployment, deflation, zero nominal interest rates and excess reserves held by commercial banks. These phenomena are commonly observed during the Great Depression in the United States, the recent long-run stagnation in Japan, and the worldwide financial crisis triggered by the US subprime loan problem of 2008. We show that an excessive liquidity preference leads to a liquidity trap and thereby generates the phenomena.

Growth, Stagnation and Status Preference

We consider three objects of people's status preference, consumption, physical capital holding and money holding, and show that an economy grows or stagnates depending on which object people most seriously take as status. If the main object of status preference is consumption, a steady state with full employment is reached. If it is physical capita (viz. a producible asset), permanent growth with full employment occurs. However, if it is money (viz. an unproducible asset), stagnation with persistent unemployment arises.