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
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
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