a(n) = floor( tan(m*Pi/2) ), where m = 1 - 2^(-n).

Terms

a(0) =1a(1) =2a(2) =5a(3) =10a(4) =20a(5) =40a(6) =81a(7) =162a(8) =325a(9) =651a(10) =1303a(11) =2607a(12) =5215a(13) =10430a(14) =20860a(15) =41721a(16) =83443a(17) =166886a(18) =333772a(19) =667544a(20) =1335088a(21) =2670176a(22) =5340353a(23) =10680707a(24) =21361414a(25) =42722829a(26) =85445659a(27) =170891318a(28) =341782637a(29) =683565275