COMPOSE natural numbers with primes.

Terms

a(0) =2a(1) =11a(2) =53a(3) =237a(4) =1013a(5) =4196a(6) =16992a(7) =67647a(8) =265743a(9) =1032827a(10) =3979023a(11) =15217248a(12) =57835016a(13) =218636365a(14) =822691425a(15) =3083074193a(16) =11512489353a(17) =42851360088a(18) =159043175322a(19) =588767623587