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 A280892 Squareful numbers with both neighbors squarefree. 2

%I

%S 4,12,16,18,20,32,36,40,52,54,56,60,68,72,84,88,90,92,96,104,108,112,

%T 128,132,140,144,150,156,160,162,164,180,184,192,196,198,200,204,212,

%U 216,220,228,232,234,236,240,248,250,252,256,264,268,270,272,284,292,294,300,304,306,308,312

%N Squareful numbers with both neighbors squarefree.

%C Subsequence of A013929 and A067874.

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

%o (MAGMA) [n: n in [2..300] | not IsSquarefree(n) and IsSquarefree(n-1) and IsSquarefree(n+1)];

%o (PARI) isok(n) = !issquarefree(n) && issquarefree(n-1) && issquarefree(n+1); \\ _Michel Marcus_, Jun 18 2017

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

%K nonn,easy

%O 1,1

%A _Juri-Stepan Gerasimov_, Jan 16 2017

%E Definition corrected by _Jon E. Schoenfield_, Jun 18 2017

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Last modified August 8 06:27 EDT 2020. Contains 336290 sequences. (Running on oeis4.)