a(n) = Sum_{k=1..n} 2^k * phi(k), where phi is the Euler totient function A000010.

Terms

a(0) =2a(1) =6a(2) =22a(3) =54a(4) =182a(5) =310a(6) =1078a(7) =2102a(8) =5174a(9) =9270a(10) =29750a(11) =46134a(12) =144438a(13) =242742a(14) =504886a(15) =1029174a(16) =3126326a(17) =4699190a(18) =14136374a(19) =22524982a(20) =47690806a(21) =89633846a(22) =274183222a(23) =408400950a(24) =1079489590a(25) =1884795958a(26) =4300715062a(27) =7521940534a(28) =22554326070a(29) =31144260662