<Dl> <Dd> cos (2π / 17). </Dd> </Dl> <Dd> cos (2π / 17). </Dd> <P> Each of those is a root of a quadratic equation in terms of the one before . Moreover, these equations have real rather than complex roots, so in principle can be solved by geometric construction: this is because the work all goes on inside a totally real field . </P> <P> 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 . </P>

Only regular polygons with an odd number of sides