% Chapter 13 - Three-Dimensional Autonomous Systems and Chaos.
% Programs_13d - Lyapunov exponents of the Lorenz system.
% Copyright Birkhauser 2004. Stephen Lynch.
% Special thanks to Vasiliy Govorukhin for allowing me to use his M-files.
% For continuous and discrete systems see the Lyapunov Exponents Toolbox of
% Steve Siu at the mathworks/matlabcentral/fileexchange.
% Reference.
% A. Wolf, J. B. Swift, H. L. Swinney, and J. A. Vastano, "Determining Lyapunov Exponents from a Time Series," Physica D,
% Vol. 16, pp. 285-317, 1985.
% You must read the above paper to understand how the program works.
% Lyapunov exponents for the Lorenz system below are:
% L_1 = 0.9022, L_2 = 0.0003, L_3 = -14.5691 when tend=10,000.
function [Texp,Lexp]=lyapunov(n,rhs_ext_fcn,fcn_integrator,tstart,stept,tend,ystart,ioutp);
n=3;rhs_ext_fcn=@lorenz_ext;fcn_integrator=@ode45;
tstart=0;stept=0.5;tend=300;
ystart=[1 1 1];ioutp=10;
n1=n; n2=n1*(n1+1);
% Number of steps.
nit = round((tend-tstart)/stept);
% Memory allocation.
y=zeros(n2,1); cum=zeros(n1,1); y0=y;
gsc=cum; znorm=cum;
% Initial values.
y(1:n)=ystart(:);
for i=1:n1 y((n1+1)*i)=1.0; end;
t=tstart;
% Main loop.
for ITERLYAP=1:nit
% Solutuion of extended ODE system.
[T,Y] = feval(fcn_integrator,rhs_ext_fcn,[t t+stept],y);
t=t+stept;
y=Y(size(Y,1),:);
for i=1:n1
for j=1:n1 y0(n1*i+j)=y(n1*j+i); end;
end;
% Construct new orthonormal basis by Gram-Schmidt.
znorm(1)=0.0;
for j=1:n1 znorm(1)=znorm(1)+y0(n1*j+1)^2; end;
znorm(1)=sqrt(znorm(1));
for j=1:n1 y0(n1*j+1)=y0(n1*j+1)/znorm(1); end;
for j=2:n1
for k=1:(j-1)
gsc(k)=0.0;
for l=1:n1 gsc(k)=gsc(k)+y0(n1*l+j)*y0(n1*l+k); end;
end;
for k=1:n1
for l=1:(j-1)
y0(n1*k+j)=y0(n1*k+j)-gsc(l)*y0(n1*k+l);
end;
end;
znorm(j)=0.0;
for k=1:n1 znorm(j)=znorm(j)+y0(n1*k+j)^2; end;
znorm(j)=sqrt(znorm(j));
for k=1:n1 y0(n1*k+j)=y0(n1*k+j)/znorm(j); end;
end;
% Update running vector magnitudes.
for k=1:n1 cum(k)=cum(k)+log(znorm(k)); end;
% Normalize exponent.
for k=1:n1
lp(k)=cum(k)/(t-tstart);
end;
% Output modification.
if ITERLYAP==1
Lexp=lp;
Texp=t;
else
Lexp=[Lexp; lp];
Texp=[Texp; t];
end;
for i=1:n1
for j=1:n1
y(n1*j+i)=y0(n1*i+j);
end;
end;
end;
% Show the Lyapunov exponent values on the graph.
str1=num2str(Lexp(nit,1));str2=num2str(Lexp(nit,2));str3=num2str(Lexp(nit,3));
plot(Texp,Lexp);
title('Dynamics of Lyapunov Exponents');
text(235,1.5,'\lambda_1=','Fontsize',10);
text(250,1.5,str1);
text(235,-1,'\lambda_2=','Fontsize',10);
text(250,-1,str2);
text(235,-13.8,'\lambda_3=','Fontsize',10);
text(250,-13.8,str3);
xlabel('Time'); ylabel('Lyapunov Exponents');
% End of plot
function f=lorenz_ext(t,X);
%
% Values of parameters.
SIGMA = 10; R = 28; BETA = 8/3;
x=X(1); y=X(2); z=X(3);
Y= [X(4), X(7), X(10);
X(5), X(8), X(11);
X(6), X(9), X(12)];
f=zeros(9,1);
%Lorenz equation.
f(1)=SIGMA*(y-x);
f(2)=-x*z+R*x-y;
f(3)=x*y-BETA*z;
%Linearized system.
Jac=[-SIGMA, SIGMA, 0;
R-z, -1, -x;
y, x, -BETA];
%Variational equation.
f(4:12)=Jac*Y;
%Output data must be a column vector.
% End of Programs_13d.