A124268 - OEIS

%I #7 Mar 19 2020 18:36:49

%S 19,37,61,71,103,107,113,181,193,197,229,239,307,317,337,349,379,383,

%T 397,479,521,523,557,571,601,619,641,643,683,691,733,787,853,857,883,

%U 887,971,977,1013,1019,1021,1033,1039,1091,1109,1123,1151,1187,1279

%N Primes indexed by 3-almost primes.

%C 3-almost primes indexed by primes = A124269. prime(3almostprime(n)) - 3almostprime(prime(n)) = A124270. See also A106349 Primes indexed by semiprimes. See also A106350 Semiprimes indexed by primes. See also A122824 Prime(semiprime(n)) - semiprime(prime(n)). Commutator [A000040,A001358] at n.

%H Giovanni Resta, <a href="/A124268/b124268.txt">Table of n, a(n) for n = 1..10000</a>

%F a(n) = prime(3almostprime(n)) = A000040(A014612(n)). {p such that p is prime and omega(primepi(p)) = 3} = {p such that p is in A000040 and A001222(A000720(p)) = 3}.

%e a(1) = prime(3almostprime(1)) = prime(8) = 19.

%e a(2) = prime(3almostprime(2)) = prime(12) = 37.

%e a(3) = prime(3almostprime(3)) = prime(18) = 61.

%t Prime[#]&/@Select[Range[400],PrimeOmega[#]==3&] (* _Harvey P. Dale_, Mar 19 2020 *)

%Y Cf. A000040, A014612, A122824, A124269, A124270.

%K easy,nonn

%O 1,1

%A _Jonathan Vos Post_, Oct 23 2006