a(n) = phi(2^n) - phi(n^2), with Euler's totient function phi = A000010.

Terms

a(0) =0a(1) =0a(2) =-2a(3) =0a(4) =-4a(5) =20a(6) =22a(7) =96a(8) =202a(9) =472a(10) =914a(11) =2000a(12) =3940a(13) =8108a(14) =16264a(15) =32640a(16) =65264a(17) =130964a(18) =261802a(19) =524128a(20) =1048324a(21) =2096932a(22) =4193798a(23) =8388416a(24) =16776716a(25) =33554120a(26) =67108378a(27) =134217392a(28) =268434644a(29) =536870672