a(n) is the number of n-digit proper prime powers.

Terms

a(0) =3a(1) =7a(2) =15a(3) =26a(4) =57a(5) =128a(6) =319a(7) =849a(8) =2285a(9) =6395a(10) =18072a(11) =51914a(12) =150497a(13) =439554a(14) =1292568a(15) =3819778a(16) =11341738a(17) =33806234a(18) =101113152a(19) =303345648a(20) =912494104a(21) =2751564993a(22) =8315282765a(23) =25179029388a(24) =76381806785a(25) =232094778772a(26) =706331084162