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A280892 Squareful numbers with both neighbors squarefree. 3
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 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Subsequence of A013929 and A067874.

LINKS

Harvey P. Dale, Table of n, a(n) for n = 1..1000

EXAMPLE

4 is in this sequence because 4 = 2^2 is nonsquarefree and both 4 - 1 = 3 and 4 + 1 = 5 are squarefree.

MATHEMATICA

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 *)

PROG

(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

CROSSREFS

Cf. A013929, A067874, A228649, A272799, A281192.

Sequence in context: A137257 A144976 A119622 * A328840 A255411 A340929

Adjacent sequences:  A280889 A280890 A280891 * A280893 A280894 A280895

KEYWORD

nonn,easy

AUTHOR

Juri-Stepan Gerasimov, Jan 16 2017

EXTENSIONS

Definition corrected by Jon E. Schoenfield, Jun 18 2017

STATUS

approved

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Last modified March 3 19:31 EST 2021. Contains 341762 sequences. (Running on oeis4.)