Constructible polygon
From Academic Kids

In mathematics, a constructible polygon is a regular polygon that can be constructed with compass and straightedge. For example, a regular pentagon is constructible with compass and straightedge while a regular heptagon is not.
Contents 
Conditions for constructibility
Some regular polygons are easy to construct with compass and straightedge; others are not. This led to the question being posed: is it possible to construct all regular ngons with compass and straightedge? If not, which ngons are constructible and which are not?
Carl Friedrich Gauss proved the constructibility of the regular 17gon in 1796. Five years later, he developed the theory of Gaussian periods in his Disquisitiones Arithmeticae. This theory allowed him to formulate a sufficient condition for the constructibility of regular polygons:
 A regular ngon can be constructed with compass and straightedge if n is the product of a power of 2 and any number of distinct Fermat primes.
Gauss conjectured that this condition was also necessary, but he offered no proof of this fact, which was proved by Pierre Wantzel in (1836). It seems very unlikely that Gauss had a correct proof, because by taking n = 9, one can immediately deduce the impossibility of trisecting an angle of 120°, a fact of which Gauss was certainly aware.
General theory
In the light of later work on Galois theory, the principles of these proofs have been clarified. It is straightforward to show from analytic geometry that constructible lengths must come from base lengths by the solution of some sequence of quadratic equations. In terms of field theory, such lengths must be contained in a field extension generated by a tower of quadratic extensions. It follows that a field generated by constructions will always have degree over the base field that is a power of two.
In the specific case of a regular ngon, the question reduces to the question of constructing a length
 cos(2π/n).
This number lies in the nth cyclotomic field — and in fact in its real subfield, which is a totally real field of degree over the rational numbers
 ½φ(n)
where φ(n) is Euler's totient function. Wantzel's result comes down to a calculation showing that φ(n) is a power of 2 precisely in the cases specified.
As for the construction of Gauss, when the Galois group is 2group it follows that it has a sequence of subgroups of orders
 1, 2, 4, 8, ...
that are nested, each in the next (a composition series, in group theory terms), something simple to prove by induction in this case of an abelian group. Therefore there are subfields nested inside the cyclotomic field, each of degree 2 over the one before. Generators for each such field can be written down by Gaussian period theory. For example for n = 17 there is a period that is a sum of eight roots of unity, one that is a sum of four roots of unity, and one that is the sum of two, which is
 cos(2π/17).
Each of those is a root of a quadratic equation in terms of the one before. Moreover these equations have real rather than imaginary roots, so in principle can be solved by geometric construction: this because the work all goes on inside a totally real field.
In this way the result of Gauss can be understood in current terms; for actual calculation of the equations to be solved, the periods can be squared and compared with the 'lower' periods, in a quite feasible algorithm.
Detailed results in terms of Fermat primes
Only five Fermat primes are known:
 F_{0} = 3, F_{1} = 5, F_{2} = 17, F_{3} = 257, and F_{4} = 65537
 Template:OEIS.
Thus an ngon is constructible if
 n = 3, 4, 5, 6, 8, 10, 12, 15, 16, 17, 20, 24, ...
 Template:OEIS,
while and an ngon is not constructible with compass and straightedge if
 n = 7, 9, 11, 13, 14, 18, 19, 21, 22, 23, 25,...
 Template:OEIS.
Compassandstraightedge constructions
Compassandstraightedge constructions are known for all constructible polygons. If n = p·q with p = 2 or p and q coprime, an ngon can be constructed from a pgon and a qgon.
 If p = 2, draw a qgon and bisect one of its central angles. From this, a 2qgon can be constructed.
 If p > 2, inscribe a pgon and a qgon in the same circle in such a way that they share a vertex. Because p and q are relatively prime, there are two vertices a central angle 360°/(p·q) apart. From this, a p·qgon can be constructed.
Thus one only has to find a compassandstraightedge construction for ngons where n is a Fermat prime.
 The construction for an equilateral triangle is simple and has been known since Antiquity. See equilateral triangle.
 Constructions for the regular pentagon were described both by Euclid (Elements, ca 300 BC), and by Ptolemy (Almagest, ca AD 150). See pentagon.
 Although Gauss proved that the regular 17gon is constructible, he didn't actually show how to do it. The first construction is due to Erchinger, a few years after Gauss' work. See heptadecagon.
 The first explicit construction of a regular 257gon was given by F.J. Richelot (1832).
 A construction for a regular 65537gon was first given by J. Hermes (1894). The construction is very complex; Hermes spent 10 years completing the 200page manuscript. (John Conway has cast doubt on the validity of Hermes' construction, however.)
Other constructions
It should be stressed that the concept of constructibility as discussed in this article applies specifically to compassandstraightedge constructions. More constructions become possible if other tools are allowed. The socalled neusis constructions, for example, make use of a marked ruler. The construction of a regular heptagon is then easy.
See also
External links
 Regular Polygon Formulas (http://mathforum.org/dr.math/faq/formulas/faq.regpoly.html), Ask Dr. Math FAQ.
 Why Gauss could not have proved necessity of constructible regular polygons (http://www.mathcs.cmsu.edu/~mjms/1996.2/clements.ps)