%
%  Modified Nicholson - Bailey
%
%  graphical identification of steady state
%  and phase-plane representation of 1st 20 
%  generations
%
%  nicba(a,r,K)    e.g.   nicba(.15,.5,14.47)
%
function nicba(a,r,K)
P = .01:.01:3;
N = (K/r)*(r-a*P);
plot(P,N)
hold on
N = P./(1-exp(-a*P));
plot(P,N,'r')
xlabel('P','fontsize',16)
ylabel('N','fontsize',16)

[P,N] = ginput(1);
gen = 20;
text(P,N,'0')

for g=1:gen
   nextN = N*exp(r*(1-N/K)-a*P);
   nextP = N*(1-exp(-a*P));
   N = nextN;
   P = nextP;
   text(P,N,num2str(g))
end
hold off
return