function retval = F(x,t)
% usage: y = F(x,t);
% description: Returns ordinate vector of values of F(x,t) 
% at abscissa vector x, scalar time t.

% local variables:
% nn: length of vector x
% k: loop variable

nn = length(x);
retval = zeros(size(x));
for k = 1:nn
  retval(k) = 0;
%  retval(k) = 2*exp(-x(k))*(-t*t+t-1+exp(-t));
end;
