a(n) = 2^(n-1)*(2^n - (-1)^n)/3.

Terms

a(0) =0a(1) =1a(2) =2a(3) =12a(4) =40a(5) =176a(6) =672a(7) =2752a(8) =10880a(9) =43776a(10) =174592a(11) =699392a(12) =2795520a(13) =11186176a(14) =44736512a(15) =178962432a(16) =715816960a(17) =2863333376a(18) =11453202432a(19) =45813071872a(20) =183251763200a(21) =733008101376