a(n) = |{m : multiplicative order of 10 mod m is equal to n}|.

Terms

a(0) =3a(1) =3a(2) =5a(3) =6a(4) =9a(5) =53a(6) =9a(7) =36a(8) =12a(9) =33a(10) =9a(11) =186a(12) =21a(13) =33a(14) =111a(15) =144a(16) =9a(17) =564a(18) =3a(19) =330a(20) =239a(21) =273a(22) =3a(23) =1756a(24) =84a(25) =165a(26) =76a(27) =714a(28) =93a(29) =16167