TOPICS
Search

Cevian Simplex


A Cevian simplex, for n>=2, corresponding to an interior point M of an n-simplex A_1A_2...A_(n+1) is the n-simplex N_1N_2...N_(n+1) whose ith polytope vertex N_i is the intersection of the line A_iM with the (n-1)-dimensional face opposite A_i (Aliyev 2026).

If M has normalized barycentric coordinates (lambda_1,lambda_2,...,lambda_(n+1)), where sum_(i=1)^(n+1)lambda_i=1, then N_i has barycentric coordinates

 N_i=1/(1-lambda_i)(lambda_1,...,lambda_(i-1),0,lambda_(i+1),...,lambda_(n+1)).
(1)

Writing V(X_1,...,X_(n+1)) for the volume of an n-simplex, the volume ratio is

 (V(N_1,...,N_(n+1)))/(V(A_1,...,A_(n+1)))=nproduct_(i=1)^(n+1)(lambda_i)/(1-lambda_i).
(2)

The sharp volume bound is

 V(N_1,...,N_(n+1))<=1/(n^n)V(A_1,...,A_(n+1)),
(3)

(Reutter and Leuenberger 1964, Klamkin 1971, Aliyev 2026), with equality iff M is the geometric centroid of A_1A_2...A_(n+1).

The n-simplex MN_1N_2...N_n is one of the n+1 component n-simplexes into which M partitions the Cevian simplex. Its volume ratio is

 (V(M,N_1,...,N_n))/(V(A_1,...,A_(n+1)))=lambda_(n+1)product_(i=1)^n(lambda_i)/(1-lambda_i).
(4)

Aliyev (2026) proved the sharp bound

 (V(M,N_1,...,N_n))/(V(A_1,...,A_(n+1)))<=((n-1)^2)/((n-theta_n)^(n+3)),
(5)

where

 theta_n=(n+1-sqrt(n^2+2n-3))/2.
(6)

Equality holds when M has barycentric coordinates (theta_n,theta_n,...,theta_n,1-ntheta_n) in the indicated order.

For n=2, the component bound reduces to

 (V(M,N_1,N_2))/(V(A_1,A_2,A_3))<=1/(phi^5),
(7)

where phi is the golden ratio. For n=3, it becomes

 (V(M,N_1,N_2,N_3))/(V(A_1,A_2,A_3,A_4))<=4/((1+sqrt(3))^6).
(8)

See also

Barycentric Coordinates, Cevian, Cevian Triangle, Geometric Centroid, Simplex

Explore with Wolfram|Alpha

References

Aliyev, Y. N. "The Maximum of the Volume of a Cevian Simplex and Its Parts." Amer. Math. Monthly, 1-5, 2026. https://doi.org/10.1080/00029890.2026.2702819.Klamkin, M. S. "A Volume Inequality for Simplexes." Publikacije Elektrotehničkog Fakulteta. Serija Matematika i Fizika 357/380, 3-4, 1971. https://www.jstor.org/stable/43667532.Reutter, O. and Leuenberger, F. "Aufgabe 454." Elem. Math. 19, 63-64, 1964. https://www.e-periodica.ch/digbib/view?pid=edm-001%3A1964%3A19%3A%3A55#69.

Cite this as:

Weisstein, Eric W. "Cevian Simplex." From MathWorld--A Wolfram Resource. https://mathworld.wolfram.com/CevianSimplex.html

Subject classifications