Conic-Constructible Regular Polygons

[John Conway]

On Wed, 15 Dec 1999, John Conway wrote:

>    But solving a quartic can be done by solving a cubic and some
> quadratics, so we can instead regard the new move as solving a cubic.
> Now it's "obvious" (if you know Galois theory)  that the degree of any
> equation that can be solved by quadratics must have form  2^a.3^b

   but meant, of course, to write  "solved by quadratics and cubics".

   JHC