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Search: id:A127202
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| A127202 |
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a(1)=1, a(2)=2; a(n) = the smallest positive integer not occurring earlier in the sequence such that GCD(a(n),a(n-1)) does not equal GCD(a(n-1),a(n-2)). |
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+0
2
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| 1, 2, 4, 3, 6, 5, 10, 7, 14, 8, 9, 12, 11, 22, 13, 26, 15, 18, 16, 17, 34, 19, 38, 20, 21, 24, 23, 46, 25, 30, 27, 28, 32, 29, 58, 31, 62, 33, 36, 35, 40, 37, 74, 39, 42, 41, 82, 43, 86, 44, 45, 48, 47, 94, 49, 56, 50, 51, 54, 52, 53, 106, 55, 60, 57, 59, 118, 61, 122, 63, 66
(list; graph; listen)
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OFFSET
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1,2
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COMMENT
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This sequence appears to be a permutation of the positive integers.
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EXAMPLE
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GCD(a(7),a(8)) = GCD(10,7) = 1. So a(9) is the smallest positive integer which does not occur earlier in the sequence and which is such that GCD(a(9), 7) is not 1. So a(9) = 14, since GCD(14,7) = 7.
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MATHEMATICA
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f[l_List] := Block[{k = 1, c = GCD[l[[ -1]], l[[ -2]]]}, While[MemberQ[l, k] || GCD[k, l[[ -1]]] == c, k++ ]; Append[l, k]]; Nest[f, {1, 2}, 69] (*Chandler*)
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CROSSREFS
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Cf. A127203.
Sequence in context: A119618 A113321 A082560 this_sequence A034701 A091857 A132340
Adjacent sequences: A127199 A127200 A127201 this_sequence A127203 A127204 A127205
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KEYWORD
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nonn
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AUTHOR
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Leroy Quet (qq-quet(AT)mindspring.com), Jan 08 2007
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EXTENSIONS
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Extended by Ray Chandler (rayjchandler(AT)sbcglobal.net), Jan 16 2007
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