Demienneract (9-demicube)
Petrie polygon
Type	Uniform 9-polytope
Family	demihypercube
Coxeter symbol	161
Schläfli symbol	{3,36,1} = h{4,37} s{21,1,1,1,1,1,1,1}
Coxeter-Dynkin diagram	=
8-faces	274	18 {31,5,1} 256 {37}
7-faces	2448	144 {31,4,1} 2304 {36}
6-faces	9888	672 {31,3,1} 9216 {35}
5-faces	23520	2016 {31,2,1} 21504 {34}
4-faces	36288	4032 {31,1,1} 32256 {33}
Cells	37632	5376 {31,0,1} 32256 {3,3}
Faces	21504	{3}
Edges	4608
Vertices	256
Vertex figure	Rectified 8-simplex
Symmetry group	D9, [36,1,1] = [1+,4,37] [28]+
Dual	?
Properties	convex

In geometry, a demienneract or 9-demicube is a uniform 9-polytope, constructed from the 9-cube, with alternated vertices removed. It is part of a dimensionally infinite family of uniform polytopes called demihypercubes.

E. L. Elte identified it in 1912 as a semiregular polytope, labeling it as HM₉ for a 9-dimensional half measure polytope.

Coxeter named this polytope as 1₆₁ from its Coxeter diagram, with a ring on one of the 1-length branches, and Schläfli symbol ${\displaystyle \left\{3{\begin{array}{l}3,3,3,3,3,3\\3\end{array))\right\))$ or {3,3^6,1}.

Cartesian coordinates

Cartesian coordinates for the vertices of a demienneract centered at the origin are alternate halves of the enneract:

(±1,±1,±1,±1,±1,±1,±1,±1,±1)

with an odd number of plus signs.

Images

orthographic projections

Coxeter plane	B9	D9	D8
Graph
Dihedral symmetry	[18]+ = [9]	[16]	[14]
Graph
Coxeter plane	D7	D6
Dihedral symmetry	[12]	[10]
Coxeter group	D5	D4	D3
Graph
Dihedral symmetry	[8]	[6]	[4]
Coxeter plane	A7	A5	A3
Graph
Dihedral symmetry	[8]	[6]	[4]