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Search: id:A138666
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| A138666 |
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Numbers n such that every sum of consecutive positive numbers ending in n is not prime. |
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+0
2
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| 1, 8, 14, 18, 20, 25, 26, 28, 32, 33, 35, 38, 39, 44, 46, 48, 50, 56, 58, 60, 62, 63, 65, 68, 72, 74, 77, 78, 80, 81, 85, 86, 88, 92, 93, 94, 95, 98, 102, 104, 105, 108, 110, 111, 116, 118, 119, 122, 123, 124, 125, 128, 130, 133, 134, 138, 140, 143, 144, 145, 146, 148
(list; graph; listen)
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OFFSET
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1,2
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COMMENT
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Also numbers n such that all terms in row n of A087401 are not prime. Also the index of the triangular numbers in A076768. See A087572 for the least prime, if it exists. David Wasserman points out (in A087572) that n is in this sequence if and only if n and 2n-1 are both not prime. This sequence is infinite because 2k^2 is a term for all k>1.
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LINKS
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T. D. Noe, Table of n, a(n) for n=1..1000
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EXAMPLE
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8 is in this sequence because 8, 15=7+8, 21=6+7+8, 26=5+6+7+8, 30=4+5+6+7+8, 33=3+4+5+6+7+8, 35=2+3+4+5+6+7+8 and 36=1+2+3+4+5+6+7+8 are all composite.
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MATHEMATICA
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Select[Range[200], !PrimeQ[ # ] && !PrimeQ[2#-1] &]
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CROSSREFS
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Sequence in context: A050681 A084021 A114456 this_sequence A072835 A096229 A112277
Adjacent sequences: A138663 A138664 A138665 this_sequence A138667 A138668 A138669
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KEYWORD
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nice,nonn
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AUTHOR
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T. D. Noe (noe(AT)sspectra.com), Mar 26 2008
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