a(n) = (2^n - (-1+i)^n - (-1-i)^n)/4 - 1 where i is the imaginary unit.

Terms

a(0) =0a(1) =0a(2) =0a(3) =5a(4) =5a(5) =15a(6) =35a(7) =55a(8) =135a(9) =255a(10) =495a(11) =1055a(12) =2015a(13) =4095a(14) =8255a(15) =16255a(16) =32895a(17) =65535a(18) =130815a(19) =262655a(20) =523775a(21) =1048575a(22) =2098175a(23) =4192255a(24) =8390655a(25) =16777215a(26) =33550335a(27) =67117055a(28) =134209535a(29) =268435455