Double geometric series in two complex variables — the canonical Hartogs-series example that converges on the full bidisk D²(4) = {|z₁|, |z₂| < 4} and nowhere else. Sympy closed form: Σ_{j,k≥0} z₁ʲz₂ᵏ/4^{j+k} = [Σ(z₁/4)ʲ]·[Σ(z₂/4)ᵏ] =…
Double geometric series in two complex variables — the canonical Hartogs-series example that converges on the full bidisk D²(4) = {|z₁|, |z₂| < 4} and nowhere else. Sympy closed form: Σ_{j,k≥0} z₁ʲz₂ᵏ/4^{j+k} = [Σ(z₁/4)ʲ]·[Σ(z₂/4)ᵏ] =…