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% Math 3520: Differential Equations and Dynamical Systems
%  Function to compute Cobweb graph coordinates
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function [X,Y] = cobweb(F,X0,steps)
% Input
%  F  = function handle, computes X(n+1) from X(n)
%  X0 = initial value X(1)
%  steps = number of visits to X=Y line (e.g., 10)
% Output
%  X,Y = coordinates to plot a cobweb starting at X0
  X(1) = X0;  % Initial value for iteration
  Y(1) = 0;   % Initial cobweb point is on X-axis
  for j=2:2:2*steps  % roundtrip to X=Y takes 2 steps
    X(j) = X(j-1);   % up/down to the graph of F
    Y(j) = F(X(j));  % 
    X(j+1) = Y(j);   % left/right to X=Y line
    Y(j+1) = Y(j);   % 
  end
end