a(n) = Sum_{k=0..n} binomial(n,k) * gcd(n,k).

Terms

a(0) =2a(1) =6a(2) =12a(3) =28a(4) =40a(5) =144a(6) =140a(7) =536a(8) =864a(9) =2560a(10) =2068a(11) =12720a(12) =8216a(13) =45192a(14) =78660a(15) =182832a(16) =131104a(17) =933984a(18) =524324a(19) =3698240a(20) =4890648a(21) =13345816a(22) =8388652a(23) =67390464a(24) =60129600a(25) =225470544a(26) =279938160a(27) =1032462256a(28) =536870968a(29) =5018059200