A108181 - OEIS

%I #19 Sep 08 2022 08:45:19

%S 9,21,25,33,49,57,65,69,77,85,93,121,129,133,141,145,161,169,177,185,

%T 201,205,209,213,217,221,237,249,253,265,289,301,305,309,321,329,341,

%U 361,365,377,381,393,413,417,437,445,453,469,473,481,485,489,493,497

%N Semiprimes of the form 4n + 1.

%C Either a(n)=(4*i+1)*(4*j+1) or a(n)=(4*i+3)*(4*j+3); - _Reinhard Zumkeller_, Jun 15 2005

%C A107978 is a subsequence. - _Reinhard Zumkeller_, Jun 15 2005

%H Vincenzo Librandi, <a href="/A108181/b108181.txt">Table of n, a(n) for n = 1..1000</a>

%H K. Ford, Jason Sneed, <a href="http://dx.doi.org/10.1080/10586458.2010.10390630">Chebyshev's Bias for products of two primes</a>, Exper. Math. 19 (4) (2010) 385-398

%t Select[4Range[0, 150] + 1, PrimeOmega[#] == 2&] (* _Vincenzo Librandi_, Sep 22 2012 *)

%o (Magma) IsSemiprime:= func<n | &+[d[2]: d in Factorization(n)] eq 2>; [s: n in [2..150] | IsSemiprime(s) where s is 4*n + 1]; // _Vincenzo Librandi_, Sep 22 2012

%Y Cf. A080774, A001358, A002144, A002145, A107978, A121387.

%K easy,nonn

%O 1,1

%A _Giovanni Teofilatto_, Jun 14 2005

%E Corrected and extended by _Reinhard Zumkeller_, Jun 15 2005