function [F]=faxis(nt,dt)
% code by: Ruiqing He, University of Utah, 2004
if mod(nt,2) 
    F=[0:(nt+1)/2-1,-(nt+1)/2+1:-1]*2*pi/(nt*dt); 
else
    F=[0:nt/2-1,0,-nt/2+1:-1]*2*pi/(nt*dt); 
end

