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Search: id:A074170
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| A074170 |
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Start with 1, add the next number if one gets a prime then add the next number else subtract the next... |
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+0
3
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| 1, 3, 6, 2, 7, 13, 20, 12, 3, 13, 24, 12, -1, -15, -30, -46, -63, -81, -100, -120, -141, -163, -140, -164, -189, -215, -242, -270, -299, -329, -360, -392, -425, -459, -494, -530, -567, -605, -644, -684, -725, -767, -810, -854, -899, -945, -992, -1040, -1089, -1139, -1190, -1242, -1295, -1349, -1404, -1460
(list; graph; listen)
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OFFSET
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1,2
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COMMENT
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Note that a(22) = -163 is the last prime generated by this sequence. All subsequent terms are composite and equal (16-n)(n+17)/2.
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FORMULA
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a(n) = -(n-16)(n+17)/2 for n > 22
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EXAMPLE
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a(1) = 1, a(2) = 1+2 =3 is a prime hence a(3) = 3 +3 = 6 which is composite hence a(4) = 6-4 = 2 etc.
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MATHEMATICA
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a=3; Join[{1, 3}, Table[If[PrimeQ[a], a=a+n, a=a-n], {n, 3, 60}]]
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CROSSREFS
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Cf. A074171.
Sequence in context: A078783 A125717 A065232 this_sequence A076543 A005132 A064388
Adjacent sequences: A074167 A074168 A074169 this_sequence A074171 A074172 A074173
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KEYWORD
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easy,sign
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AUTHOR
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Amarnath Murthy (amarnath_murthy(AT)yahoo.com), Aug 30 2002
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EXTENSIONS
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Corrected and extended by Jason Earls (zevi_35711(AT)yahoo.com), Sep 01 2002
Corrected by T. D. Noe (noe(AT)sspectra.com), Oct 04 2004
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