isoperimetrische

Isoperimetrische, or isoperimetric, refers to problems and inequalities that compare boundary size to enclosed content. In mathematics, an isoperimetric problem asks which shape or region minimizes boundary measure for a given volume or, equivalently, maximizes volume for a given boundary length. The core idea is that symmetry often yields optimality.

In the Euclidean plane, the classic result states that among all closed curves of fixed length L, the maximum possible enclosed area A is A ≤ L^2/(4π), with equality precisely for a circle. Equivalently, among figures with area A, the least possible perimeter P is P ≥ 2√(πA). These equivalences highlight the circle as the optimal shape in two dimensions.

In higher dimensions, the analogous statement says that among shapes with a given volume, the surface area is minimized by a ball. Formally, for a region Ω in n-dimensional space with volume |Ω| and boundary measure (surface area) |∂Ω|, the isoperimetric inequality states |∂Ω| ≥ n ω_n^{1/n} |Ω|^{(n-1)/n}, where ω_n is the volume of the unit ball. Equality occurs for balls.

The isoperimetric principle extends to curved spaces and to discrete settings such as graphs, where one looks at the balance between boundary size and contained volume. In Riemannian geometry, isoperimetric inequalities interact with curvature and lead to isoperimetric profiles and constants (for example, the Cheeger constant).

Historically, isoperimetric questions trace to ancient Greek geometry, with the circle identified as optimal in the plane. The term combines Greek roots meaning “equal measure.” The topic remains active across geometry, analysis, and applied fields.