A046412 - OEIS

A046412

Lengths of nonsquarefree repunits.

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