Pi/9 - sqrt((1 - sin(Pi/18))/8).
2, 7, 6, 7, 2, 0, 4, 5, 5, 5, 5, 5, 9, 6, 2, 5, 2, 2, 2, 3, 4, 1, 6, 4, 4, 8, 7, 4, 4, 0, 9, 0, 8, 2, 6, 2, 3, 4, 7, 9, 4, 4, 3, 5, 6, 0, 5, 5, 0, 4, 1, 9, 6, 0, 5, 6, 9, 4, 9, 6, 6, 0, 6, 9, 8, 6, 4, 5, 4, 0, 4, 8, 0, 0, 2, 1, 0, 5, 6, 8, 8, 8, 1, 2, 0, 4, 2, 0, 0, 0, 6, 8, 1, 0, 4, 7, 2, 0, 6, 3, 2, 3, 8, 6, 9, 9, 7
-1
When an 9-gon is inscibed in a unit circle, this is the area of one of the nine segments of the circle not in the 9-gon.
T. D. Noe, Plot of 1000 terms
T. D. Noe, Table of 1000 terms
Eric W. Weisstein, Circular segment
Wikipedia, Circular segment
This is Pi/n - sqrt((p^2+q^2)*((p-1)^2+q^2)), where p = (1 + cos(2*Pi/n))/2 and q = sin(2*Pi/n)/2 for n=9.
The number is 0.0276720455555962522234164487440908262347944356055….
(Mma) RealDigits[Pi/9 - Sqrt[(1 - Sin[Pi/18])/8], 10, 105][[1]]
Cf. S000236, S000237, S000238, S000243-S000249, S000250, S000251.
nonn,cons
T. D. Noe, Sep 05 2014