A086686
Number of 1<=i
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, 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, 6, 33, 8, 18, 14, 15, 10, 27, 10, 21, 13, 29, 10, 29, 3, 18, 19
Offset: 1
Keywords
Examples
For n=2, i=2 gives a prime (3), so a(2)=1. For n=10, i=2,3,6,8,9 give primes, so a(10)=5.
Programs
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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))) -
PARI
a(n) = sum(i=1, n, isprime(i*n-1)); \\ Michel Marcus, Mar 17 2014
Extensions
a(2) corrected and more terms from Michel Marcus, Mar 17 2014