It is ENNEAGON, ENNEAGONAL

[Antreas P. Hatzipolakis]

%N A048910 Indices of nonagonal numbers which are also square.
%H A048910 Source
%N A048911 Indices of square numbers which are also nonagonal.
%H A048911 Source
%C A049013 These polygons fill in many of the gaps left by the Greeks, who were
restricted to compass and ruler. Specifically, they make possible construction
of the regular 7-sided heptagon, 9-sided nonagon, 11-gon and 13-gon. The 14-gon
becomes the first to be impossible by either ruler, compass or knotting.
%H A028991 Source
%N A048912 Nonagonal square numbers.
%H A048912 Source
%N A048907 Indices of nonagonal numbers which are also triangular.
%H A048907 Source
%N A048916 Indices of nonagonal numbers which are also hexagonal.
%H A048916 Source
%N A048917 Indices of hexagonal numbers which are also nonagonal.
%H A048917 Source
%N A048913 Indices of nonagonal numbers which are also pentagonal.
%H A048913 Source
%N A048914 Indices of pentagonal numbers which are also nonagonal.
%H A048914 Source
%H A028992 Source
%N A048908 Indices of triangular numbers which are also nonagonal.
%H A048908 Source
%N A048919 Indices of nonagonal numbers which are also heptagonal.
%H A048919 Source
%N A048920 Indices of heptagonal numbers which are also nonagonal.
%H A048920 Source
%N A048909 Nonagonal triangular numbers.
%H A048909 Source
%N A048918 Nonagonal hexagonal numbers.
%H A048918 Source
%N A048922 Indices of nonagonal numbers which are also octagonal.
%H A048922 Source
%N A048923 Indices of octagonal numbers which are also nonagonal.
%H A048923 Source
%N A048915 Nonagonal pentagonal numbers.
%H A048915 Source
%N A048921 Nonagonal heptagonal numbers.
%H A048921 Source
%N A048924 Nonagonal octagonal numbers.
%H A048924 Source