Number of holes in a sheet of paper when you fold it n times and cut off the four corners.

Terms

a(0) =0a(1) =0a(2) =1a(3) =3a(4) =9a(5) =21a(6) =49a(7) =105a(8) =225a(9) =465a(10) =961a(11) =1953a(12) =3969a(13) =8001a(14) =16129a(15) =32385a(16) =65025a(17) =130305a(18) =261121a(19) =522753a(20) =1046529a(21) =2094081a(22) =4190209a(23) =8382465a(24) =16769025a(25) =33542145a(26) =67092481a(27) =134193153a(28) =268402689a(29) =536821761