a(n) = Sum_{k=1..n} floor(n/k) * 4^(k-1).

Terms

a(0) =1a(1) =6a(2) =23a(3) =92a(4) =349a(5) =1394a(6) =5491a(7) =21944a(8) =87497a(9) =349902a(10) =1398479a(11) =5593892a(12) =22371109a(13) =89484074a(14) =357919803a(15) =1431678080a(16) =5726645377a(17) =22906581142a(18) =91626057879a(19) =366504227292