OFFSET
0,1
FORMULA
Equals Product_{k>=2} (1 - 1/(3*k*(k - 1)/2 + 1)).
Equals Product_{k>=2} (1 - 1/A005448(k)).
EXAMPLE
0.5419123258325301299411023662126578003950258848928...
MATHEMATICA
RealDigits[2 Pi Sech[Sqrt[5/3] Pi/2]/3, 10, 120][[1]]
PROG
(PARI) 2*Pi*(1/cosh(sqrt(5/3)*Pi/2))/3 \\ Michel Marcus, Jun 20 2019
CROSSREFS
KEYWORD
nonn,cons
AUTHOR
Ilya Gutkovskiy, Jun 19 2019
STATUS
approved