function [u] = sect_4_prob_2_u_from_x_N_lambda(t,x,lambda, u0,x0,n,a,b,c)
%   
% Written by:
% -- 
% John L. Weatherwax                2006-08-28
% 
% email: wax@alum.mit.edu
% 
% Please send comments and especially bug reports to the
% above email address.
% 
%-----

num = - b * u0 * ( lambda .* exp( c * t ) );
den = ( 1 - (x/x0).^2 );
u   = -u0 * log( num ./ den );