 Isoperimetric inequality

The isoperimetric inequality is a fundamental result in mathematics, particularly in geometry and analysis. It relates the length of a closed curve (the perimeter) to the area it encloses. The classic formulation states that for a simple closed curve in the plane, the perimeter \( P \) and the area \( A \) are related by the inequality: \[ P^2 \geq 4\pi A, \] with equality holding if and only if the shape is a circle.