a(n) = floor(1/(Pi - 2^(n+1)*sin(Pi/2^(n+1)))).

Terms

a(0) =3a(1) =12a(2) =49a(3) =198a(4) =792a(5) =3170a(6) =12681a(7) =50727a(8) =202909a(9) =811636a(10) =3246545a(11) =12986183a(12) =51944732a(13) =207778928a(14) =831115713a(15) =3324462855a(16) =13297851421a(17) =53191405684a(18) =212765622737a(19) =851062490950