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

Terms

a(0) =1a(1) =15a(2) =81a(3) =240a(4) =610a(5) =1230a(6) =2336a(7) =3840a(8) =6371a(9) =9455a(10) =14097a(11) =19615a(12) =27441a(13) =36205a(14) =48849a(15) =61874a(16) =79860a(17) =99470a(18) =124816a(19) =150846a(20) =186498a(21) =221646a(22) =267232a(23) =313840a(24) =373059a(25) =431599a(26) =508595a(27) =581009a(28) =673635a(29) =767835