Computing Second-Order-Accurate Solutions for Rational Expectation Models Using Linear Solution Methods

This paper shows how to compute a second-order accurate solution of a non-linear rational expectation model using algorithms developed for the solution of linear rational expectation models. This result is a state-space representation for the realized values of the variables of the model. This state-space representation can easily be used to compute impulse responses as well as conditional and unconditional forecasts.Second-order approximation; solution method for rational expectation models.

A hypothetical upper bound on the heights of the solutions of a Diophantine equation with a finite number of solutions

Let f(1)=1, and let f(n+1)=2^{2^{f(n)}} for every positive integer n. We conjecture that if a system S \subseteq {x_i \cdot x_j=x_k: i,j,k \in {1,...,n}} \cup {x_i+1=x_k: i,k \in {1,...,n}} has only finitely many solutions in non-negative integers x_1,...,x_n, then each such solution (x_1,...,x_n) satisfies x_1,...,x_n \leq f(2n). We prove: (1) the conjecture implies that there exists an algorithm which takes as input a Diophantine equation, returns an integer, and this integer is greater than the heights of integer (non-negative integer, positive integer, rational) solutions, if the solution set is finite, (2) the conjecture implies that the question whether or not a Diophantine equation has only finitely many rational solutions is decidable with an oracle for deciding whether or not a Diophantine equation has a rational solution, (3) the conjecture implies that the question whether or not a Diophantine equation has only finitely many integer solutions is decidable with an oracle for deciding whether or not a Diophantine equation has an integer solution, (4) the conjecture implies that if a set M \subseteq N has a finite-fold Diophantine representation, then M is computable.Comment: 13 pages, section 7 expande

Spherical wave diffraction by a rational wedge

In this paper we derive a new expression for the point source Green's function for the reduced wave equation, valid in an angular sector, whoseangle is equal to a rational multiple of . This Green's function is used to find new expressions for the field produced by the diffraction of a spherical wave source by a wedge, whose angle can be expressed as a rational multiple of . The expressions obtained are in the form of source terms and real integrals which represent the diffracted field. The general resultobtained includes as special cases Macdonald’s solution for diffraction by ahalf plane; a solution for the problem of diffraction by a wedge of open angle 3/2, i.e. a corner; a new representation for the solution of the problemof diffraction by a mixed soft/hard half plane; and a new representation for the point source Green's function for Laplace's equation, valid in an angularsector whose angle is equal to a rational multiple of