function [ut]=derv(n,t,u)
%
% Function derv computes the derivative vector 
% of the 2 x 2 ode problem
%
  global ncalls;
  a=5.5;
  b=4.5;
  ut(1)=-a*u(1)+b*u(2);
  ut(2)= b*u(1)-a*u(2);
  ncalls=ncalls+1;
