Duality (order theory)

In geometry, Pedoe's inequality, named after Daniel Pedoe, states that if a, b, and c are the lengths of the sides of a triangle with area ƒ, and A, B, and C are the lengths of the sides of a triangle with area F, then

$A^{2}(b^{2}+c^{2}-a^{2})+B^{2}(a^{2}+c^{2}-b^{2})+C^{2}(a^{2}+b^{2}-c^{2})\geq 16Ff,\,$

with equality if and only if the two triangles are similar.

The expression on the left is not only symmetric under any of the six permutations of the set { (A, a), (B, b), (C, c) } of pairs, but also—perhaps not so obviously—remains the same if a is interchanged with A and b with B and c with C. In other words, it is a symmetric function of the pair of triangles.

Pedoe's inequality is a generalization of Weitzenböck's inequality and of the Hadwiger–Finsler inequality.

References

• "A Two-Triangle Inequality", Daniel Pedoe, The American Mathematical Monthly, volume 70, number 9, page 1012, November, 1963.
• "An Inequality for Two Triangles", D. Pedoe, Proceedings of the Cambridge Philosophical Society, volume 38, part 4, page 397, 1943.

External links

• Pedoe's inequality