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A046412 Lengths of nonsquarefree repunits. 4
9, 18, 22, 27, 36, 42, 44, 45, 54, 63, 66, 72, 78, 81, 84, 88, 90, 99, 108, 110, 111, 117, 126, 132, 135, 144, 153, 154, 156, 162, 168, 171, 176, 180, 189, 198, 205, 207, 210, 216, 220, 222, 225, 234, 242, 243, 252, 261, 264, 270, 272, 279, 286, 288, 294, 297, 306, 308, 312, 315, 324, 330, 333, 336, 342, 351, 352 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

This is the set of all positive multiples of all positive members of A087094. What is the asymptotic density of this set? - Jeppe Stig Nielsen, Dec 28 2015

LINKS

Table of n, a(n) for n=1..67.

P. De Geest, Repunits prime factors

Eric Weisstein's World of Mathematics, Repunit

FORMULA

a(n)=k where (10^k-1)/9 is not squarefree. - Ray Chandler, Aug 10 2003

MAPLE

remove(t -> numtheory:-issqrfree((10^t-1)/9), [$1..90]); # Robert Israel, Dec 30 2015

MATHEMATICA

Select[Range[300], !SquareFreeQ[(10^#-1)/9]&] (* Harvey P. Dale, Jun 26 2013 *)

PROG

(PARI) isok(n) = ! issquarefree((10^n-1)/9); \\ Michel Marcus, Dec 31 2015

CROSSREFS

Cf. A000042, A004022, A046053, A084006, A087094.

Sequence in context: A306382 A222623 A141469 * A171081 A232056 A109661

Adjacent sequences:  A046409 A046410 A046411 * A046413 A046414 A046415

KEYWORD

nonn,base

AUTHOR

Patrick De Geest, Jul 15 1998

EXTENSIONS

Terms to a(60) from Ray Chandler, Aug 10 2003

a(61)-a(67) from Max Alekseyev, Apr 29 2022

STATUS

approved

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Last modified May 27 03:13 EDT 2022. Contains 354093 sequences. (Running on oeis4.)