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A016105 Blum integers: numbers of the form p * q where p and q are distinct primes congruent to 3 (mod 4). 12
21, 33, 57, 69, 77, 93, 129, 133, 141, 161, 177, 201, 209, 213, 217, 237, 249, 253, 301, 309, 321, 329, 341, 381, 393, 413, 417, 437, 453, 469, 473, 489, 497, 501, 517, 537, 553, 573, 581, 589, 597, 633, 649, 669, 681, 713, 717, 721, 737, 749, 753, 781, 789 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Subsequence of A084109. - Ralf Stephan and David W. Wilson, Apr 17 2005

Subsequence of A046388. - Altug Alkan, Dec 10 2015

Subsequence of A339817. No common terms with A339870. - Antti Karttunen, Dec 26 2020

LINKS

Antti Karttunen, Table of n, a(n) for n = 1..26828; all terms < 2^19 (first 1000 terms from T. D. Noe)

Joe Hurd, Blum Integers, Talk at the Trinity College, Jan 20 1997.

Wikipedia, Blum integer

FORMULA

a(n) = A195758(n) * A195759(n). - Reinhard Zumkeller, Sep 23 2011

MAPLE

N:= 10000: # to get all terms <= N

Primes:= select(isprime, [seq(i, i=3..N/3, 4)]):

S:=select(`<=`, {seq(seq(Primes[i]*Primes[j], i=1..j-1), j=2..nops(Primes))}, N):

sort(convert(S, list)); # Robert Israel, Dec 11 2015

MATHEMATICA

With[{upto = 820}, Select[Union[Times@@@Subsets[ Select[Prime[Range[ PrimePi[ NextPrime[upto/3]]]], Mod[#, 4] == 3 &], {2}]], # <= upto &]] (* Harvey P. Dale, Aug 19 2011 *)

Select[4Range[5, 197] + 1, PrimeNu[#] == 2 && MoebiusMu[#] == 1 && Mod[FactorInteger[#][[1, 1]], 4] != 1 &] (* Alonso del Arte, Nov 18 2015 *)

PROG

(Haskell)

import Data.Set (singleton, fromList, deleteFindMin, union)

a016105 n = a016105_list !! (n-1)

a016105_list = f [3, 7] (drop 2 a002145_list) 21 (singleton 21) where

   f qs (p:p':ps) t s

     | m < t     = m : f qs (p:p':ps) t s'

     | otherwise = m : f (p:qs) (p':ps) t' (s' `union` (fromList pqs))

     where (m, s') = deleteFindMin s

           t' = head $ dropWhile (> 3*p') pqs

           pqs = map (p *) qs

-- Reinhard Zumkeller, Sep 23 2011

(Perl) use ntheory ":all"; forcomposites { say if ($_ % 4) == 1 && is_square_free($_) && scalar(factor($_)) == 2 && !scalar(grep { ($_ % 4) != 3 } factor($_)); } 10000; # Dana Jacobsen, Dec 10 2015

(PARI) list(lim)=my(P=List(), v=List(), t, p); forprime(p=2, lim\3, if(p%4==3, listput(P, p))); for(i=2, #P, p=P[i]; for(j=1, i-1, t=p*P[j]; if(t>lim, break); listput(v, t))); Set(v) \\ Charles R Greathouse IV, Jul 01 2016

(PARI) isA016105(n) = (2==omega(n)&&2==bigomega(n)&&1==(n%4)&&3==((factor(n)[1, 1])%4)); \\ Antti Karttunen, Dec 26 2020

CROSSREFS

Cf. A002145, A006881, A046388, A339870.

Intersection of A005117 and A107978.

Also, subsequence of the following sequences: A046388, A084109, A091113, A167181, A339817.

Sequence in context: A190299 A280262 A084109 * A187073 A271101 A191683

Adjacent sequences:  A016102 A016103 A016104 * A016106 A016107 A016108

KEYWORD

nonn,easy,nice

AUTHOR

Robert G. Wilson v

EXTENSIONS

More terms from Erich Friedman

STATUS

approved

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Last modified February 26 11:15 EST 2021. Contains 341631 sequences. (Running on oeis4.)