A272491 Decimal expansion of the edge length of a regular 19-gon with unit circumradius.

3, 2, 9, 1, 8, 9, 1, 8, 0, 5, 6, 1, 4, 6, 7, 7, 8, 8, 2, 8, 7, 3, 0, 4, 1, 1, 8, 1, 7, 5, 8, 7, 6, 8, 3, 9, 0, 2, 4, 3, 4, 4, 9, 6, 6, 7, 1, 9, 3, 0, 8, 2, 4, 6, 7, 0, 2, 9, 4, 2, 5, 4, 8, 0, 9, 8, 1, 5, 3, 8, 0, 5, 7, 0, 4, 9, 4, 3, 4, 1, 2, 5, 9, 5, 5, 7, 4, 6, 2, 8, 7, 6, 0, 1, 8, 7, 9, 8, 6, 0, 7, 7, 2, 8, 5

Offset: 0

Author

Stanislav Sykora, May 01 2016

Keywords

• nonn
• cons
• easy

Comments

The edge length e(m) of a regular m-gon is e(m) = 2*sin(Pi/m). In this case, m = 19, and the constant, a = e(19), is not constructible using a compass and a straightedge (see A004169). With an odd m, in fact, e(m) would be constructible only if m were a Fermat prime (A019434).

Examples

			0.32918918056146778828730411817587683902434496671930824670294254...
		

Links

• Stanislav Sykora, Table of n, a(n) for n = 0..2000
• Wikipedia, Constructible number
• Wikipedia, Regular polygon
• Index entries for algebraic numbers, degree 18

Crossrefs

Cf. A004169, A019434.

Edge lengths of nonconstructible n-gons: A272487 (n=7), A272488 (n=9), A272489 (n=11), A272490 (n=13), A255241 (n=14), A130880 (n=18).

Programs

Mathematica

RealDigits[N[2Sin[Pi/19], 100]][[1]] (* Robert Price, May 01 2016 *)

PARI

2*sin(Pi/19)

Formula

Equals 2*sin(Pi/19) = 2*cos(Pi*17/38).