a(n) = Sum_{d|n} tau(d^n), where tau() is the number of divisors (A000005).

Terms

a(0) =1a(1) =4a(2) =5a(3) =15a(4) =7a(5) =64a(6) =9a(7) =52a(8) =30a(9) =144a(10) =13a(11) =546a(12) =15a(13) =256a(14) =289a(15) =165a(16) =19a(17) =1140a(18) =21a(19) =1386a(20) =529a(21) =576a(22) =25a(23) =3848a(24) =78a(25) =784a(26) =166a(27) =2610a(28) =31a(29) =32768