a(n) = (1/n)*Sum_{k=1..n} k*2^gcd(n,k).

Terms

a(0) =2a(1) =5a(2) =10a(3) =20a(4) =36a(5) =74a(6) =134a(7) =272a(8) =526a(9) =1052a(10) =2058a(11) =4160a(12) =8204a(13) =16466a(14) =32824a(15) =65696a(16) =131088a(17) =262490a(18) =524306a(19) =1049168a(20) =2097316a(21) =4195358a(22) =8388630a(23) =16779632a(24) =33554516a(25) =67112996a(26) =134218282a(27) =268443848a(28) =536870940a(29) =1073759432