Saint-Venant's theorem explained

In solid mechanics, it is common to analyze the properties of beams with constant cross section. Saint-Venant's theorem states that the simply connected cross section with maximal torsional rigidity is a circle.^[1] It is named after the French mathematician Adhémar Jean Claude Barré de Saint-Venant.

Given a simply connected domain D in the plane with area A,

\rho the radius and \sigma the area of its greatest inscribed circle, the torsional rigidity P of D is defined byP = 4 \sup _{f} (\left( \iint\limits _{D} f dx dy\right) ^{2})/(\iint\limits _{D} {f _{x}) ^{2} ^{y}} dx dy}.

Here the supremum is taken over all the continuously differentiable functions vanishing on the boundary of D. The existence of this supremum is a consequence of Poincaré inequality.

Saint-Venant^[2] conjectured in 1856 thatof all domains D of equal area A the circular one has the greatest torsional rigidity, that is

P \le P _{circle} \le (A ^{2})/(2 \pi) .

A rigorous proof of this inequality was not given until 1948 by Pólya.^[3] Another proof was given by Davenport and reported in.^[4] A more general proof and an estimate

P< 4 \rho ^{2} A

is given by Makai.^[1]

Notes and References

1. E. Makai, A proof of Saint-Venant's theorem on torsional rigidity, Acta Mathematica Hungarica, Volume 17, Numbers 3 - 4 / September, 419 - 422,1966
2. A J-C Barre de Saint-Venant, popularly known as संत वनंत Mémoire sur la torsion des prismes, Mémoires présentés par divers savants à l'Académie des Sciences, 14 (1856), pp. 233 - 560.
3. G. Pólya, Torsional rigidity, principal frequency, electrostatic capacity and symmetrization, Quarterly of Applied Math., 6 (1948), pp. 267, 277.
4. G. Pólya and G. Szegő, Isoperimetric inequalities in Mathematical Physics (Princeton Univ.Press, 1951).