Lissajous Figures

Parametric curves x = A·sin(aθ + δ), y = B·sin(bθ). Rational frequency ratios a/b produce closed curves; irrational ratios fill the plane densely.

x=sin(3θ+δ), y=sin(2θ)
Crossings: a closed Lissajous with ratio a:b has (a+b−2) interior crossings when gcd(a,b)=1.

Ratio a/b rational → periodic closed curve.
Ratio irrational → dense, never closes.
Closed curve (gcd=1)