

A130880


Decimal expansion of 2*sin(Pi/18).


10



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

0,1


COMMENTS

Also: a bond percolation threshold probability on the triangular lattice.
Also: the edge length of a regular 18gon with unit circumradius. Such an mgon is not constructible using a compass and a straightedge (see A004169). With an even m, in fact, it would be constructible only if the (m/2)gon were constructible, which is not true in this case (see A272488).  Stanislav Sykora, May 01 2016


LINKS

Stanislav Sykora, Table of n, a(n) for n = 0..2000
S. R. Finch, Several Constants Arising in Statistical Mechanics, Annals Combinat. vol 3 (1999) issue (24) pp. 323335.
Wikipedia, Constructible number
Wikipedia, Regular polygon


FORMULA

Equals 2*A019819 = A019829/A019889.


EXAMPLE

0.34729635533386069770....


MATHEMATICA

RealDigits[N[2Sin[Pi/18], 100]][[1]] (* Robert Price, May 01 2016 *)


PROG

(PARI) 2*sin(Pi/18)


CROSSREFS

Cf. A004169, A019819.
Edge lengths of nonconstructible ngons: A271487 (n=7), A272488 (n=9), A272489 (n=11), A272490 (n=13), A255241 (n=14), A272491 (n=19).  Stanislav Sykora, May 01 2016
Sequence in context: A230159 A105828 A209873 * A026248 A082089 A089961
Adjacent sequences: A130877 A130878 A130879 * A130881 A130882 A130883


KEYWORD

cons,nonn


AUTHOR

R. J. Mathar, Jul 26 2007


STATUS

approved



