OFFSET
1,1
COMMENTS
Composite numbers k such that 10^k == 10 (mod 9k).
If k is a term, then so is (10^k - 1)/9. Thus, the sequence is infinite.
No terms are divisible by 3. - Robert Israel, May 28 2018
LINKS
Robert Israel, Table of n, a(n) for n = 1..4130
EXAMPLE
(10^10 - 1)/9 = 1111111111 == 1 (mod 10), so the composite 10 is a term.
Equivalently, we have the congruence 10^10 == 10 (mod 9*10).
MAPLE
filter:= n -> n mod 3 <> 0 and (10&^n - 10) mod n = 0\ and not isprime(n):
select(filter, [$4..10^5]); # Robert Israel, May 28 2018
MATHEMATICA
Select[Range@ 20000, ! PrimeQ@# && PowerMod[10, #, 9 #] == 10 &] (* Robert G. Wilson v, Apr 28 2018 *)
PROG
(PARI) isok(n) = (n>1) && !isprime(n) && Mod(10, 9*n)^n == 10; \\ Michel Marcus, Apr 28 2018
CROSSREFS
KEYWORD
nonn
AUTHOR
Thomas Ordowski, Apr 27 2018
EXTENSIONS
a(4) onward from Robert G. Wilson v, Apr 27 2018
STATUS
approved