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 A086686 Number of 1<=i

%I

%S 0,1,2,3,1,5,2,4,3,5,2,8,1,7,5,7,3,10,4,9,6,10,3,13,6,8,7,11,5,19,6,9,

%T 7,9,6,18,5,13,9,16,5,22,7,16,13,12,9,19,7,16,11,15,8,21,12,16,14,14,

%U 6,33,8,18,14,15,10,27,10,21,13,29,10,29,3,18,19

%N Number of 1<=i<n such that i*n-1 is prime.

%e For n=2, i=2 gives a prime (3), so a(2)=1.

%e For n=10, i=2,3,6,8,9 give primes, so a(10)=5.

%o (PARI) nphi(n)=local(c); c=0; for (i=1,n-1,if (isprime(i*n-1),c++)); c for(i=1,60,print1(","nphi(i)))

%o (PARI) a(n) = sum(i=1, n, isprime(i*n-1)); \\ _Michel Marcus_, Mar 17 2014

%K nonn

%O 1,3

%A _Jon Perry_, Jul 28 2003

%E a(2) corrected and more terms from _Michel Marcus_, Mar 17 2014

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Last modified August 3 19:05 EDT 2021. Contains 346441 sequences. (Running on oeis4.)