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A047845
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(n-1)/2, where n runs through odd nonprimes (A014076).
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15
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0, 4, 7, 10, 12, 13, 16, 17, 19, 22, 24, 25, 27, 28, 31, 32, 34, 37, 38, 40, 42, 43, 45, 46, 47, 49, 52, 55, 57, 58, 59, 60, 61, 62, 64, 66, 67, 70, 71, 72, 73, 76, 77, 79, 80, 82, 84, 85, 87, 88, 91, 92, 93, 94, 97, 100, 101, 102, 103, 104, 106, 107, 108, 109, 110, 112, 115
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,2
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COMMENTS
| Also (starting with 2nd term) numbers of the form 2xy+x+y for x and y positive integers. This is also the numbers of sticks needed to construct a two dimensional rectangular lattice of unit squares. See A090767 for the three-dimensional generalization. - John Mason (j.h.mason(AT)open.ac.uk), Feb 02 2004
Note that if k is not in a(n), then 2*k+1 is prime. - Jose Brox (tautocrona(AT)terra.es), Dec 29 2005
Values of n for which A073610(2n+3)=0; Values of n for which A061358(2n+3)=0; - Graeme McRae (g_m(AT)mcraefamily.com), Jul 18 2006
Comment from Andrzej Staruszkiewicz (uszkiewicz(AT)poczta.onet.pl): This sequence also arises in the following way: take the product of initial odd numbers i.e. the product (2n+1)!/(n!2^n) and factor it into prime numbers. The result will be of the form 3^n(3)5^n(5)7^n(7)11^n(11).... . Then n(3)/n(5) = 2, n(3)/n(7) = 3,n(3)/n(11) = 5,... and this sequence forms (for sufficiently large n, of course) the sequence of natural numbers without 4,7,10,12,... i.e. these numbers are what is lacking in the present sequence.
Let p odd prime, number in sequence, n=(p^2-1)/2 (4,12,24,60,84,...) [From Vincenzo Librandi (vincenzo.librandi(AT)tin.it), Aug 06 2010]
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CROSSREFS
| Complement of A005097.
Sequence in context: A000414 A025357 A144020 * A097703 A104036 A178598
Adjacent sequences: A047842 A047843 A047844 * A047846 A047847 A047848
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KEYWORD
| easy,nonn
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AUTHOR
| Enoch Haga (Enokh(AT)comcast.net)
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