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

Terms

a(0) =2a(1) =17a(2) =80a(3) =323a(4) =1214a(5) =4373a(6) =15308a(7) =52487a(8) =177146a(9) =590489a(10) =1948616a(11) =6377291a(12) =20726198a(13) =66961565a(14) =215233604a(15) =688747535a(16) =2195382770a(17) =6973568801a(18) =22082967872a(19) =69735688019a(20) =219667417262a(21) =690383311397