A329081 Decimal expansion of Sum_{k>=0} 1/(k^2-3).

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

Offset

0,1

Comments

In general, for complex numbers z, if we define F(z) = Sum_{k>=0} 1/(k^2+z), f(z) = Sum_{k>=1} 1/(k^2+z), then we have:

F(z) = (1 + sqrt(z)*Pi*coth(sqrt(z)*Pi))/(2z), z != 0, -1, -4, -9, -16, ...;

f(z) = (-1 + sqrt(z)*Pi*coth(sqrt(z)*Pi))/(2z), z != 0, -1, -4, -9, -16, ...; Pi^2/6, z = 0. Note that f(z) is continuous at z = 0.

This sequence gives F(-3).

Links

Table of n, a(n) for n=0..90.

Formula

Equals (1 + (sqrt(-3)*Pi)*coth(sqrt(-3)*Pi))/(-6) = (1 + (sqrt(3)*Pi)*cot(sqrt(3)*Pi))/(-6).

Example

0.64331685665276028377...

Mathematica

RealDigits[(1 + Sqrt[3]*Pi*Cot[Sqrt[3]*Pi])/6, 10, 120][[1]] (* Amiram Eldar, Jun 17 2023 *)

Prog

(PARI) default(realprecision, 100); my(F(x) = (1 + (sqrt(x)*Pi)/tanh(sqrt(x)*Pi))/(2*x)); F(-3)
(PARI) sumnumrat(1/(x^2-3), 0) \\ Charles R Greathouse IV, Jan 20 2022

Crossrefs

Cf. A329080 (F(-5)), this sequence (F(-3)), A329082 (F(-2)), A113319 (F(1)), A329083 (F(2)), A329084 (F(3)), A329085 (F(4)), A329086 (F(5)).

Cf. A329087 (f(-5)), A329088 (f(-3)), A329089 (f(-2)), A013661 (f(0)), A259171 (f(1)), A329090 (f(2)), A329091 (f(3)), A329092 (f(4)), A329093 (f(5)).

Sequence in context: A197760 A217290 A157296 * A343461 A155044 A245634

Adjacent sequences: A329078 A329079 A329080 * A329082 A329083 A329084

Keyword

nonn,cons

Author

Jianing Song, Nov 04 2019

Status

approved