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A280892
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Squareful numbers with both neighbors squarefree.
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3
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4, 12, 16, 18, 20, 32, 36, 40, 52, 54, 56, 60, 68, 72, 84, 88, 90, 92, 96, 104, 108, 112, 128, 132, 140, 144, 150, 156, 160, 162, 164, 180, 184, 192, 196, 198, 200, 204, 212, 216, 220, 228, 232, 234, 236, 240, 248, 250, 252, 256, 264, 268, 270, 272, 284, 292, 294, 300, 304, 306, 308, 312
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OFFSET
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1,1
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COMMENTS
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Subsequence of A013929 and A067874.
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LINKS
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Harvey P. Dale, Table of n, a(n) for n = 1..1000
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EXAMPLE
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4 is in this sequence because 4 = 2^2 is nonsquarefree and both 4 - 1 = 3 and 4 + 1 = 5 are squarefree.
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MATHEMATICA
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Mean/@SequencePosition[Table[If[SquareFreeQ[n], 1, 0], {n, 400}], {1, 0, 1}] (* Requires Mathematica version 10 or later *) (* Harvey P. Dale, Sep 19 2020 *)
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PROG
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(MAGMA) [n: n in [2..300] | not IsSquarefree(n) and IsSquarefree(n-1) and IsSquarefree(n+1)];
(PARI) isok(n) = !issquarefree(n) && issquarefree(n-1) && issquarefree(n+1); \\ Michel Marcus, Jun 18 2017
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CROSSREFS
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Cf. A013929, A067874, A228649, A272799, A281192.
Sequence in context: A137257 A144976 A119622 * A328840 A255411 A340929
Adjacent sequences: A280889 A280890 A280891 * A280893 A280894 A280895
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KEYWORD
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nonn,easy
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AUTHOR
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Juri-Stepan Gerasimov, Jan 16 2017
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EXTENSIONS
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Definition corrected by Jon E. Schoenfield, Jun 18 2017
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STATUS
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approved
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