function retval = Nsoln(r,t,M,D,a)
% usage: retval = Nsoln(r,t,M,D,a)
% description: returns solution to radial parabolic PDE
%  N_t = (1/r)*(D*r*N_r)_r + f(r,t,N), 0 < r < infinity
%
% Current form f(r,t,N) = a*N
% user must derem only one of the following functions:

% continuous line source of M units per unit time.
% See p. 348 of G&DP (CntLine of paper):
%
%retval = M*exp(a*t)*expint(r.^2/(4*D*t))/(4*pi*D);

% instantaneous point source M units for N(r,t).
%(InstPoinr of paper):
retval = M*exp(a*t-r.^2/(4*D*t))/(4*pi*D*t)^1.5;

% not used in paper, but another data generator:
% instantaneous line source of M units per unit time.
% See p. 335 of G&DP for details for InstLine:
%retval = M*exp(a*t-r.^2/(4*D*t))/(4*pi*D*t); 
