Question: Error, recursive assignment

restart;
with(plottools);
with(plots);
a := 1;
b := 1;
c := 1;
k := 1;
l := 1;
omega := 1;
A[2] = 2;
alpha := 2;
beta := 1;
kappa := 0.5;
C[1] := 1;
lambda := -1;

omega := (-alpha^6*b^4*lambda + 2*alpha^6*b^2*l^2 - 2*a^2*alpha^4*l^2 + 2*alpha^4*b^2*k^2 + a^4*alpha^2*lambda - 2*a^2*alpha^2*k^2 + 4*a*C[1])/(-4*alpha^2*b^2 + 4*a^2);

a[0] := 0;

a[1] := sqrt(-(-alpha^2*b^2 + a^2)/(4*beta))*alpha;

b[1] := sqrt(-(alpha^2*b^2*lambda*sigma - a^2*lambda*sigma)/(4*beta))*alpha;

sigma := A[1]*A[1] - A[2]*A[2];

T := A[1]*sinh(xi*sqrt(-lambda)) + A[2]*cosh(xi*sqrt(-lambda)) + mu/lambda;

t := diff(T, xi);

S := t/T;

R := 1/T;

mu := 0;

A[1] := 0;

y := 0;

xi := k*x^kappa/kappa + l*y^kappa/kappa - omega*t^kappa/kappa;

  Error, recursive assignment