a(n) = binomial(2^n-1,2).

Terms

a(0) =0a(1) =0a(2) =3a(3) =21a(4) =105a(5) =465a(6) =1953a(7) =8001a(8) =32385a(9) =130305a(10) =522753a(11) =2094081a(12) =8382465a(13) =33542145a(14) =134193153a(15) =536821761a(16) =2147385345a(17) =8589737985a(18) =34359345153a(19) =137438167041a(20) =549754241025