a(n) = usigma(n) - n, where usigma(n) = sum of unitary divisors of n (A034448).

Terms

a(0) =0a(1) =1a(2) =1a(3) =1a(4) =1a(5) =6a(6) =1a(7) =1a(8) =1a(9) =8a(10) =1a(11) =8a(12) =1a(13) =10a(14) =9a(15) =1a(16) =1a(17) =12a(18) =1a(19) =10a(20) =11a(21) =14a(22) =1a(23) =12a(24) =1a(25) =16a(26) =1a(27) =12a(28) =1a(29) =42