A216415 - OEIS

%I #49 Feb 20 2019 09:56:17

%S 1,1,0,6,1,2,6,0,16,18,6,22,0,3,28,15,2,0,3,6,5,21,0,46,42,16,13,0,18,

%T 58,60,6,0,33,22,35,8,0,6,13,9,41,0,28,44,6,15,0,96,2,4,34,0,53,108,3,

%U 112,0,6,48,22,5,0,42,21,130,18,0,8,46,46,6,0,42,148,75,16,0,78,13,66,81,0,166,78,18,43,0,58,178,180,60,0,16,6,95,192,0,98,99

%N a(n) = smallest positive m such that 2n-1 | 10^m-1, or 0 if no such m exists.

%C This is yet another version of the sequences defined in A002329, A007732, A070682, A084680. - _N. J. A. Sloane_, Sep 08 2012

%C a(n) gives the multiplicative order of 10 mod (2n-1), if it is finite, or 0 if not defined.

%H V. Raman, <a href="/A216415/b216415.txt">Table of n, a(n) for n = 1..100</a>.

%o (PARI) for(i=0,200,i++;if(i%5==0,print1(0","),print1(znorder(Mod(10,i))","))) \\ _V. Raman_, Nov 22 2012

%o (PARI) for(i=0,200,i++;m=0;for(x=1,i,if(((10^x-1))%i==0,m=x;break));print1(m",")) \\ _V. Raman_, Nov 22 2012

%Y Cf. A002326, A002329, A007732, A070682, A084680. - _N. J. A. Sloane_, Sep 08 2012

%K nonn

%O 1,4

%A _V. Raman_, Sep 07 2012