A079147 - OEIS

A079147

Primes p such that p+1 has at most 2 prime factors, counted with multiplicity; i.e., primes p such that bigomega(p+1) = A001222(p+1) <= 2.

%I #7 May 17 2018 16:39:56

%S 2,3,5,13,37,61,73,157,193,277,313,397,421,457,541,613,661,673,733,

%T 757,877,997,1093,1153,1201,1213,1237,1321,1381,1453,1621,1657,1753,

%U 1873,1933,1993,2017,2137,2341,2473,2557,2593,2797,2857,2917,3061,3217,3253

%N Primes p such that p+1 has at most 2 prime factors, counted with multiplicity; i.e., primes p such that bigomega(p+1) = A001222(p+1) <= 2.

%C Sum of reciprocals ~ 1.266

%e 157 is in the sequence because 157 + 1 = 2*79 has 2 prime factors.

%t Select[Prime[Range[500]],PrimeOmega[#+1]<3&] (* _Harvey P. Dale_, May 17 2018 *)

%o (PARI) s(n) = {sr=0; forprime(x=2,n, if(bigomega(x+1) < 3, print1(x" "); sr+=1.0/x; ); ); print(); print(sr); } \\ Lists primes p<=n such that p+1 has at most 2 prime factors.

%Y Except for 2, this is identical to A005383. Cf. A079148, A079149, A079150.

%K easy,nonn

%O 1,1

%A _Cino Hilliard_, Dec 27 2002