OFFSET
1,7
COMMENTS
When a(n) is not zero, it is a divisor of phi(n). If n is a prime with primitive root 10 (cf. A001913) then a(n) = (n-1)/2.
LINKS
Antti Karttunen, Table of n, a(n) for n = 1..10001
EXAMPLE
a(7) = a(13) = 3 as 1001 is divisible by 7 and 13. a(17) = 8 as 17 divides 100000001 = 10^8 + 1.
PROG
(PARI) A069531(n) = { fordiv(eulerphi(n), k, if(!((1+(10^k))%n), return(k))); (0); }; \\ Antti Karttunen, Aug 23 2019
CROSSREFS
KEYWORD
nonn
AUTHOR
Amarnath Murthy, Apr 01 2002
EXTENSIONS
More terms from Vladeta Jovovic, Apr 03 2002
STATUS
approved