Why is the action not displayed?

code

Does anyone know why my story is not displayed? I get a recursion error.

ω = Range(0, 5, 1);
U = 0;
μ = 0;
Σ(ω_) := 0;
G(ω_) := 0.5*(ω + μ - Σ(ω)) + 0.5*Sqrt((ω + μ - Σ(ω)) ^2 - 4);
G0(ω_) := ((G(ω))^(-1) + Σ(ω));
Gnew(ω_) := 0.5*G0(ω) + 0.5*( (G0(ω))^(-1) - U)^(-1);
Σ(ω_) := (G0(ω))^(-1) - (G(ω))^(-1);

Plot(Im(G(ω)), {ω, 0, 5})