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A088782 a(n) = smallest k>0 such that concatenation of n^k and 1 is prime, or 0 if no such number exists. 3

%I

%S 1,2,1,1,2,1,1,2,2,1,10,0,1,6,1,3,1356,1,1,2,1,15,0,1,1,2,1,1,4,2,1,0,

%T 1,0,2,3,2,4,4,1,2,1,1,6,0,1,2,2,1,4,3,1,16,1,9,0,1,2,36,1,165,66,1,1,

%U 0,1,0,6,1,1,2,12,3,138,1,1,4,0,5,4,1,1,2,5,2,2,3,1,0,2,1,24,2,1,42,7,1,0,1

%N a(n) = smallest k>0 such that concatenation of n^k and 1 is prime, or 0 if no such number exists.

%C a(185) > 10^6, see link. _Richard N. Smith_, Jul 16 2019

%H Richard N. Smith, <a href="/A088782/b088782.txt">Table of n, a(n) for n = 1..184</a>

%H Karsten Bonath, <a href="https://www.rieselprime.de/ziki/Proth_prime_small_bases_least_n">Proth prime small bases least n</a>

%H Richard N. Smith, <a href="/A088782/a088782.txt">Table of n, a(n) for n = 1..500 status</a>

%t f[n_] := Block[{k = 1}, While[ !PrimeQ[10*n^k + 1], k++ ]; k]]; f[1] = 1; Table[ f[n], {n, 1, 99}] (* _Robert G. Wilson v_, Oct 29 2003 *)

%o (PARI) a(n)=if((n%11==1 || n%33==32) && n>1, 0, for(k=1, 10^6, if(ispseudoprime(10*n^k+1), return(k)))) \\ _Richard N. Smith_, Jul 16 2019

%Y Cf. A088622, A088783.

%K base,nonn

%O 1,2

%A _Ray Chandler_, Oct 23 2003

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Last modified June 6 04:15 EDT 2020. Contains 334859 sequences. (Running on oeis4.)