A073249 Nonprime squarefree numbers n such that both n-1 and n+1 are not squarefree and not prime.

26, 51, 55, 91, 161, 170, 235, 249, 295, 305, 339, 341, 362, 377, 413, 415, 451, 485, 489, 530, 551, 559, 579, 595, 629, 638, 649, 651, 665, 667, 685, 687, 703, 721, 723, 737, 749, 849, 851, 874, 917, 926, 949, 951, 955, 962, 989, 1015, 1027, 1057, 1059

Offset: 1

Author

Reinhard Zumkeller, Jul 22 2002

Keywords

• nonn

Links

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

Crossrefs

Cf. A000469, A005117, A073247, A073250, A073251.

Programs

Mathematica

npsQ[n_]:=SquareFreeQ[n]&&NoneTrue[n+{1,0,-1},PrimeQ]&&NoneTrue[n+{1,-1},SquareFreeQ]; Select[Range[2000],npsQ] (* Requires Mathematica version 10 or later *) (* Harvey P. Dale, Jan 20 2019 *)
Mean/@SequencePosition[Table[Which[CompositeQ[n]&&SquareFreeQ[n],1,!SquareFreeQ[ n] && CompositeQ[ n],-1,True,0],{n,1100}],{-1,1,-1}] (* Harvey P. Dale, Jun 17 2022 *)

Extensions

Corrected by Harvey P. Dale, Jan 20 2019