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A016105
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Blum numbers: of form P*Q where P&Q are distinct primes congruent to 3 (mod 4).
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6
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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; internal format)
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OFFSET
| 1,1
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COMMENTS
| A subset of A084109. - Ralf Stephan and David W. Wilson, Apr 17 2005.
a(n) = A195758(n) * A195759(n). [Reinhard Zumkeller, Sep 23 2011]
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LINKS
| T. D. Noe, Table of n, a(n) for n=1..1000
Wikipedia, Blum integer
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MATHEMATICA
| With[{upto=820}, Select[Union[Times@@@Subsets[Select[Prime[Range[PrimePi[ NextPrime[upto/3]]]], Mod[#, 4]==3&], {2}]], #<=upto&]] (* From Harvey P. Dale, Aug 19 2011 *)
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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
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CROSSREFS
| Cf. A002145, A006881.
Sequence in context: A189986 A190299 A084109 * A187073 A191683 A032603
Adjacent sequences: A016102 A016103 A016104 * A016106 A016107 A016108
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KEYWORD
| nonn,easy,nice
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AUTHOR
| Robert G. Wilson v (rgwv(AT)rgwv.com)
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EXTENSIONS
| More terms from Erich Friedman (erich.friedman(AT)stetson.edu).
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