1,7

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.

Antti Karttunen, Table of n, a(n) for n = 1..10001

a(7) = a(13) = 3 as 1001 is divisible by 7 and 13. a(17) = 8 as 17 divides 100000001 = 10^8 + 1.

(PARI) A069531(n) = { fordiv(eulerphi(n), k, if(!((1+(10^k))%n), return(k))); (0); }; \\ Antti Karttunen, Aug 23 2019

Cf. A069521 to A069530.

Cf. A000010, A001913, A002329.

Sequence in context: A096693 A193139 A083206 * A035677 A143276 A283470

Adjacent sequences: A069528 A069529 A069530 * A069532 A069533 A069534

nonn

Amarnath Murthy, Apr 01 2002

More terms from Vladeta Jovovic, Apr 03 2002

approved