Conic-Constructible Regular Polygons

[John Conway]

On Tue, 14 Dec 1999, Antreas P. Hatzipolakis wrote:
> 
> Does anyone know which is the sequence of the R. polygons which are
> conic-constructible? (since right now I have not handy the Math. Int.
> volumes to check)

   I think it's obvious that the answer is those for which n has the form
2^a.3^b.p.q.r....   ,  where  p,q,r,...  are distinct primes of the
form  2^x.3^y + 1.   Yes, that IS obvious.

    John Conway