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A334880
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Numbers k such that gcd(k, k-th composite number) > 1.
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2
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2, 5, 6, 7, 10, 12, 14, 15, 16, 18, 20, 21, 22, 24, 26, 28, 30, 32, 34, 36, 42, 48, 50, 51, 52, 54, 55, 56, 60, 62, 63, 64, 65, 70, 72, 74, 75, 76, 77, 78, 80, 81, 82, 93, 96, 98, 99, 102, 104, 105, 110, 112, 114, 120, 122, 124, 126, 128, 130, 132, 138, 148
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OFFSET
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1,1
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LINKS
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EXAMPLE
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In the following table, c(k) = A002808(k) = k-th composite number.
k c(k) gcd(k, c(k))
1 4 1
2 6 2
3 8 1
4 9 1
5 10 5
6 12 6
2, 5, 6 are in this sequence, and 1,3,4 are in A073258.
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MATHEMATICA
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c = Select[Range[2, 150], ! PrimeQ[#] &]; (* A002808 *)
Select[Range[Length[c]], GCD[c[[#]], #] > 1 &] (* A334880 *)
Module[{nn=200, cmps, len}, cmps=Select[Range[nn], CompositeQ]; len=Length[ cmps]; Select[Thread[{Range[len], cmps}], GCD@@#>1&]][[All, 1]] (* Harvey P. Dale, Sep 20 2020 *)
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PROG
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(PARI) lista(nn) = {my(n=0, list=List()); forcomposite (c=1, nn, n++; if (gcd(n, c) > 1, listput(list, n))); Vec(list); } \\ Michel Marcus, Jul 19 2020
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CROSSREFS
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KEYWORD
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nonn,easy
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AUTHOR
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STATUS
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approved
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