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A019913
Decimal expansion of tangent of 15 degrees.
8
2, 6, 7, 9, 4, 9, 1, 9, 2, 4, 3, 1, 1, 2, 2, 7, 0, 6, 4, 7, 2, 5, 5, 3, 6, 5, 8, 4, 9, 4, 1, 2, 7, 6, 3, 3, 0, 5, 7, 1, 9, 4, 7, 4, 6, 1, 8, 9, 6, 1, 9, 3, 7, 1, 9, 4, 4, 1, 9, 3, 0, 2, 0, 5, 4, 8, 0, 6, 6, 9, 8, 3, 0, 9, 1, 1, 9, 9, 9, 6, 2, 9, 1, 8, 8, 5, 3, 8, 1, 3, 2, 4, 2, 7, 5, 1, 4, 2, 4
OFFSET
0,1
COMMENTS
Also, 2 - sqrt(3) = cotangent of 75 degrees. An equivalent definition of this sequence: decimal expansion of x < 1 satisfying x^2 - 4*x + 1 = 0. - Arkadiusz Wesolowski, Nov 29 2011
Multiplied by -1 (that is, -2 + sqrt(3)), this is one of three real solutions to x^3 = 15x + 4. The other two are 4 and -2 - sqrt(3), all of which can be found with Viete's formula. - Alonso del Arte, Dec 15 2012
Wentworth (1903) shows how to compute the tangent of 15 degrees to five decimal places by the laborious process of adding up the first few terms of Pi/12 + Pi^3/5184 + 2Pi^5/3732480 + 17Pi^7/11287019520 + ... - Alonso del Arte, Mar 13 2015
A quadratic integer. - Charles R Greathouse IV, Aug 27 2017
This is the radius of the largest sphere that can be placed in the space between a sphere of radius 1 and the corners of its circumscribing cube. - Amiram Eldar, Jul 11 2020
REFERENCES
Paul J. Nahin, An Imaginary Tale: The Story of sqrt(-1). Princeton, New Jersey: Princeton University Press (1988): 22 - 23.
LINKS
Willis F. Kern and James R. Bland, Solid Mensuration: With Proofs, 2nd ed., J. Wiley & Sons, Inc., New York, 1938. See pp. 91-92.
George Albert Wentworth, New Plane and Spherical Trigonometry, Surveying, and Navigation, Boston: The Atheneum Press (1903), p. 240.
FORMULA
Equals Sum_{k>=1} binomial(2*k,k)/(6^k*(k+1)). - Amiram Eldar, Jul 11 2020
Equals exp(-arccosh(2)). - Amiram Eldar, Jul 06 2023
EXAMPLE
0.2679491924311227064725536...
MATHEMATICA
RealDigits[N[Tan[15 Degree], 200]][[1]] (* Arkadiusz Wesolowski, Nov 29 2011 *)
PROG
(PARI) 2-sqrt(3) \\ Charles R Greathouse IV, Aug 27 2017
CROSSREFS
Cf. A002194 (sqrt(3)).
Sequence in context: A351088 A296443 A102046 * A299421 A327181 A047554
KEYWORD
nonn,cons
AUTHOR
STATUS
approved