OFFSET
1,2
LINKS
P. De Geest, Repunits prime factors
Eric Weisstein's World of Mathematics, Repunit
EXAMPLE
a(n) = 10 -> 11*41*271*9091 -> 1(1)*4(1)*27(1)*909(1).
MATHEMATICA
Select[Range[60], Length[fi=FactorInteger[(10^#-1)/9]]==Length[Select[First/@fi, Mod[#, 10]==1&]] &](* Ray Chandler, Apr 24 2017 *)
CROSSREFS
KEYWORD
nonn,more,base
AUTHOR
Patrick De Geest, Jul 15 1998
EXTENSIONS
a(16)-a(21) from Ray Chandler, Apr 24 2017
a(22) from Max Alekseyev, May 14 2022
Definition corrected by Max Alekseyev, Aug 19 2024
STATUS
approved