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%I #14 Sep 08 2022 08:45:17
%S 7,13,23,29,43,47,73,79,97,101,137,139,149,163,167,199,227,233,257,
%T 269,271,293,313,347,373,389,421,439,443,449,467,487,491,499,577,607,
%U 631,647,653,661,673,677,727,751,757,811,821,823,829,839,907,929,937,947
%N Primes indexed by semiprimes.
%C This is the sequence of the n-th prime for n = {4,6,9,10,14,15,21,22,25,26,33,34,35,38,39,46,49,51,...}. Not to be confused with A106350: semiprimes indexed by primes.
%H Michael De Vlieger, <a href="/A106349/b106349.txt">Table of n, a(n) for n = 1..10000</a>
%F a(n) = prime(semiprime(n)).
%F a(n) = A000040(A001358(n)).
%F pi(a(n)) = p*q for some primes p and q.
%e a(1) = 7 because semiprime(1) = 4, so prime(semiprime(1)) = prime(4) = 7.
%t Prime@ Select[Range@ 161, PrimeOmega@ # == 2 &] (* or *) Select[Prime@ Range@ 161, PrimeOmega@ PrimePi@ # == 2 &] (* _Michael De Vlieger_, Nov 28 2015 *)
%o (Magma) [NthPrime(n): n in [2..200] | &+[d[2]: d in Factorization(n)] eq 2]; // _Vincenzo Librandi_, Nov 28 2015
%o (PARI) lista(nn) = select(x->(bigomega(primepi(x))==2), primes(nn)); \\ _Michel Marcus_, Nov 29 2015
%Y Cf. A000040, A001358, A007097, A091022, A105997, A105998, A106350.
%K nonn,easy
%O 1,1
%A _Jonathan Vos Post_, Apr 29 2005
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