Number of final Tic-Tac-Toe positions on a (2*n+1) X 3 board that result in a tie.

Terms

a(0) =3a(1) =16a(2) =30a(3) =72a(4) =178a(5) =444a(6) =1114a(7) =2808a(8) =7098a(9) =17984a(10) =45656a(11) =116106a(12) =295718a(13) =754226a(14) =1926060a(15) =4924188a(16) =12602416a(17) =32284214a(18) =82777240a(19) =212415744a(20) =545495716a(21) =1401849594a(22) =3604921774a(23) =9275890122a(24) =23881602058a(25) =61518226734a(26) =158548607640a(27) =408814563524