pi(n) is a power of 2, where pi(n) = A000720(n) is the number of primes <= n.

Terms

a(0) =2a(1) =3a(2) =4a(3) =7a(4) =8a(5) =9a(6) =10a(7) =19a(8) =20a(9) =21a(10) =22a(11) =53a(12) =54a(13) =55a(14) =56a(15) =57a(16) =58a(17) =131a(18) =132a(19) =133a(20) =134a(21) =135a(22) =136a(23) =311a(24) =312a(25) =719a(26) =720a(27) =721a(28) =722a(29) =723