Enneagon, Hendecagon

[David W. Wilson]

"Antreas P. Hatzipolakis" wrote:
> 
> %N A048910 Indices of nonagonal numbers which are also square.
> %N A048911 Indices of square numbers which are also nonagonal.
> %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.
> %N A048912 Nonagonal square numbers.

Nonagon and enneagon are both correct for a 9-gon.  The situation is
even worse
for the 11-gon, variously called an undecagon, unidecagon, or
hendecagon.
All five of these names appear in my 1930's vintage Webster's
unabridged, so
they all have some historical weight.  I suspect that Conway would weigh
in on
the side of "enneagon" and "hendecagon" on the grounds of Greek purity. 
I think
that the EIS should stick with one variant, whatever it is.