A108181 Semiprimes of the form 4n + 1.

9, 21, 25, 33, 49, 57, 65, 69, 77, 85, 93, 121, 129, 133, 141, 145, 161, 169, 177, 185, 201, 205, 209, 213, 217, 221, 237, 249, 253, 265, 289, 301, 305, 309, 321, 329, 341, 361, 365, 377, 381, 393, 413, 417, 437, 445, 453, 469, 473, 481, 485, 489, 493, 497

Offset: 1

Author

Giovanni Teofilatto, Jun 14 2005

Keywords

• easy
• nonn

Comments

Either a(n)=(4*i+1)*(4*j+1) or a(n)=(4*i+3)*(4*j+3); - Reinhard Zumkeller, Jun 15 2005

A107978 is a subsequence. - Reinhard Zumkeller, Jun 15 2005

Links

• Vincenzo Librandi, Table of n, a(n) for n = 1..1000
• K. Ford, Jason Sneed, Chebyshev's Bias for products of two primes, Exper. Math. 19 (4) (2010) 385-398

Crossrefs

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

Programs

Magma

IsSemiprime:= func; [s: n in [2..150] | IsSemiprime(s) where s is 4*n + 1]; // Vincenzo Librandi, Sep 22 2012

Mathematica

Select[4Range[0, 150] + 1, PrimeOmega[#] == 2&] (* Vincenzo Librandi, Sep 22 2012 *)

Extensions

Corrected and extended by Reinhard Zumkeller, Jun 15 2005