如何计算凸包的棱

hbghlyj

The Four Pillars Of Geometry p166 介绍了 24-cell
它的顶点是
(1,0,0,0), (-1,0,0,0), (0,1,0,0), (0,-1,0,0), (0,0,1,0), (0,0,-1,0), (0,0,0,1), (0,0,0,-1),
(1/2, 1/2, 1/2, 1/2), (1/2, 1/2, 1/2, -1/2), (1/2, 1/2, -1/2, 1/2), (1/2, 1/2, -1/2, -1/2), (1/2, -1/2, 1/2, 1/2), (1/2, -1/2, 1/2, -1/2), (1/2, -1/2, -1/2, 1/2), (1/2, -1/2, -1/2, -1/2), (-1/2, 1/2, 1/2, 1/2), (-1/2, 1/2, 1/2, -1/2), (-1/2, 1/2, -1/2, 1/2), (-1/2, 1/2, -1/2, -1/2), (-1/2, -1/2, 1/2, 1/2), (-1/2, -1/2, 1/2, -1/2), (-1/2, -1/2, -1/2, 1/2), (-1/2, -1/2, -1/2, -1/2)
问题：如何算出它的棱连接了哪些点呢？

hbghlyj

(1,0,0,0) 与 8 个顶点 (1/2, ±1/2, ±1/2, ±1/2) 相连。
由 24-cell 的对称性，每个顶点都与另外 8 个顶点相连。
24 × 8/2 = 96，所以 24-cell 有 96 条棱。

1. points = {
2.     {1, 0, 0, 0}, {-1, 0, 0, 0}, {0, 1, 0, 0}, {0, -1, 0, 0},
3.     {0, 0, 1, 0}, {0, 0, -1, 0}, {0, 0, 0, 1}, {0, 0, 0, -1},
4.     {1/2, 1/2, 1/2, 1/2}, {1/2, 1/2, 1/2, -1/2}, {1/2, 1/2, -1/2, 1/2}, {1/2, 1/2, -1/2, -1/2},
5.     {1/2, -1/2, 1/2, 1/2}, {1/2, -1/2, 1/2, -1/2}, {1/2, -1/2, -1/2, 1/2}, {1/2, -1/2, -1/2, -1/2},
6.     {-1/2, 1/2, 1/2, 1/2}, {-1/2, 1/2, 1/2, -1/2}, {-1/2, 1/2, -1/2, 1/2}, {-1/2, 1/2, -1/2, -1/2},
7.     {-1/2, -1/2, 1/2, 1/2}, {-1/2, -1/2, 1/2, -1/2}, {-1/2, -1/2, -1/2, 1/2}, {-1/2, -1/2, -1/2, -1/2}
8. };
10. adj = Table[
11.    If[EuclideanDistance[points[[i]], points[[j]]] == 1, 1, 0],
12.    {i, 24}, {j, 24}
13. ];
15. Print[MatrixForm[adj]];
16. Print["Row sums: ", Total[adj, {2}]];
17. Print["Column sums: ", Total[adj]];

Copy the Code

\begin{array}{cccccccccccccccccccccccc}
0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 1 & 1 & 1 & 1 & 1 & 1 & 1 & 1 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 \\
0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 1 & 1 & 1 & 1 & 1 & 1 & 1 & 1 \\
0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 1 & 1 & 1 & 1 & 0 & 0 & 0 & 0 & 1 & 1 & 1 & 1 & 0 & 0 & 0 & 0 \\
0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 1 & 1 & 1 & 1 & 0 & 0 & 0 & 0 & 1 & 1 & 1 & 1 \\
0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 1 & 1 & 0 & 0 & 1 & 1 & 0 & 0 & 1 & 1 & 0 & 0 & 1 & 1 & 0 & 0 \\
0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 1 & 1 & 0 & 0 & 1 & 1 & 0 & 0 & 1 & 1 & 0 & 0 & 1 & 1 \\
0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 1 & 0 & 1 & 0 & 1 & 0 & 1 & 0 & 1 & 0 & 1 & 0 & 1 & 0 & 1 & 0 \\
0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 1 & 0 & 1 & 0 & 1 & 0 & 1 & 0 & 1 & 0 & 1 & 0 & 1 & 0 & 1 \\
1 & 0 & 1 & 0 & 1 & 0 & 1 & 0 & 0 & 1 & 1 & 0 & 1 & 0 & 0 & 0 & 1 & 0 & 0 & 0 & 0 & 0 & 0 & 0 \\
1 & 0 & 1 & 0 & 1 & 0 & 0 & 1 & 1 & 0 & 0 & 1 & 0 & 1 & 0 & 0 & 0 & 1 & 0 & 0 & 0 & 0 & 0 & 0 \\
1 & 0 & 1 & 0 & 0 & 1 & 1 & 0 & 1 & 0 & 0 & 1 & 0 & 0 & 1 & 0 & 0 & 0 & 1 & 0 & 0 & 0 & 0 & 0 \\
1 & 0 & 1 & 0 & 0 & 1 & 0 & 1 & 0 & 1 & 1 & 0 & 0 & 0 & 0 & 1 & 0 & 0 & 0 & 1 & 0 & 0 & 0 & 0 \\
1 & 0 & 0 & 1 & 1 & 0 & 1 & 0 & 1 & 0 & 0 & 0 & 0 & 1 & 1 & 0 & 0 & 0 & 0 & 0 & 1 & 0 & 0 & 0 \\
1 & 0 & 0 & 1 & 1 & 0 & 0 & 1 & 0 & 1 & 0 & 0 & 1 & 0 & 0 & 1 & 0 & 0 & 0 & 0 & 0 & 1 & 0 & 0 \\
1 & 0 & 0 & 1 & 0 & 1 & 1 & 0 & 0 & 0 & 1 & 0 & 1 & 0 & 0 & 1 & 0 & 0 & 0 & 0 & 0 & 0 & 1 & 0 \\
1 & 0 & 0 & 1 & 0 & 1 & 0 & 1 & 0 & 0 & 0 & 1 & 0 & 1 & 1 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 1 \\
0 & 1 & 1 & 0 & 1 & 0 & 1 & 0 & 1 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 1 & 1 & 0 & 1 & 0 & 0 & 0 \\
0 & 1 & 1 & 0 & 1 & 0 & 0 & 1 & 0 & 1 & 0 & 0 & 0 & 0 & 0 & 0 & 1 & 0 & 0 & 1 & 0 & 1 & 0 & 0 \\
0 & 1 & 1 & 0 & 0 & 1 & 1 & 0 & 0 & 0 & 1 & 0 & 0 & 0 & 0 & 0 & 1 & 0 & 0 & 1 & 0 & 0 & 1 & 0 \\
0 & 1 & 1 & 0 & 0 & 1 & 0 & 1 & 0 & 0 & 0 & 1 & 0 & 0 & 0 & 0 & 0 & 1 & 1 & 0 & 0 & 0 & 0 & 1 \\
0 & 1 & 0 & 1 & 1 & 0 & 1 & 0 & 0 & 0 & 0 & 0 & 1 & 0 & 0 & 0 & 1 & 0 & 0 & 0 & 0 & 1 & 1 & 0 \\
0 & 1 & 0 & 1 & 1 & 0 & 0 & 1 & 0 & 0 & 0 & 0 & 0 & 1 & 0 & 0 & 0 & 1 & 0 & 0 & 1 & 0 & 0 & 1 \\
0 & 1 & 0 & 1 & 0 & 1 & 1 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 1 & 0 & 0 & 0 & 1 & 0 & 1 & 0 & 0 & 1 \\
0 & 1 & 0 & 1 & 0 & 1 & 0 & 1 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 1 & 0 & 0 & 0 & 1 & 0 & 1 & 1 & 0 \\
\end{array}每行、每列之和为8，说明每个点连接了8个点

hbghlyj

从T. Banchoff的网页Schlegel Polyhedra for Regular Polytopes上24-cell的投影图，确实每个顶点有8条棱