a(n) = (2^n + 4)*(2^n - 1)/6.

Terms

a(0) =0a(1) =1a(2) =4a(3) =14a(4) =50a(5) =186a(6) =714a(7) =2794a(8) =11050a(9) =43946a(10) =175274a(11) =700074a(12) =2798250a(13) =11188906a(14) =44747434a(15) =178973354a(16) =715860650a(17) =2863377066a(18) =11453377194a(19) =45813246634a(20) =183252462250a(21) =733008800426