a(1)=1; a(n) = Sum_{1<=k<n, gcd(k,n(n+1)/2)=1} a(k).

Terms

a(0) =1a(1) =1a(2) =1a(3) =2a(4) =4a(5) =8a(6) =6a(7) =11a(8) =21a(9) =51a(10) =11a(11) =22a(12) =133a(13) =159a(14) =151a(15) =328a(16) =707a(17) =1414a(18) =880a(19) =1732a(20) =3850a(21) =9482a(22) =1742a(23) =3480a(24) =22126a(25) =37243a(26) =25604a(27) =51381a(28) =102087a(29) =204174