Where 7^n occurs in n-almost-primes, starting at a(0)=1.

Terms

a(0) =1a(1) =4a(2) =17a(3) =82a(4) =385a(5) =1688a(6) =7089a(7) =28893a(8) =115180a(9) =450906a(10) =1740244a(11) =6640747a(12) =25115604a(13) =94312569a(14) =352110321a(15) =1308256678a(16) =4841115048a(17) =17852264639a(18) =65636109307a(19) =240689877440a(20) =880582139867