A086686 - OEIS

A086686

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

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

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OFFSET

1,3

LINKS

Table of n, a(n) for n=1..75.

EXAMPLE

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.

PROG

(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

CROSSREFS

Sequence in context: A346872 A075014 A304880 * A361614 A306807 A021816

Adjacent sequences: A086683 A086684 A086685 * A086687 A086688 A086689

KEYWORD

nonn

AUTHOR

Jon Perry, Jul 28 2003

EXTENSIONS

a(2) corrected and more terms from Michel Marcus, Mar 17 2014

STATUS

approved