OFFSET
1,2
COMMENTS
Decimal expansion of the sum of the reciprocals of the centered triangular numbers (A005448).
LINKS
Eric Weisstein's World of Mathematics, Centered Triangular Number
FORMULA
Equals Sum_{k>=1} 1/(3*k*(k - 1)/2 + 1).
Equals Sum_{k>=1} 1/A005448(k).
EXAMPLE
1.56706513126405467758811157795995464399516007...
MATHEMATICA
RealDigits[2 Pi Tanh[Sqrt[5/3] Pi/2]/Sqrt[15], 10, 120][[1]]
PROG
(PARI) 2*Pi*tanh(sqrt(5/3)*Pi/2)/sqrt(15) \\ Michel Marcus, Feb 08 2019
CROSSREFS
KEYWORD
nonn,cons
AUTHOR
Ilya Gutkovskiy, Feb 07 2019
STATUS
approved