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 A086685 Number of 1 <= i < n such that i*n+1 is prime. 1
 0, 1, 1, 2, 1, 4, 2, 2, 3, 5, 3, 6, 4, 5, 5, 5, 3, 10, 2, 6, 6, 9, 4, 9, 5, 9, 7, 11, 4, 17, 3, 10, 9, 12, 9, 15, 4, 9, 11, 13, 5, 21, 7, 11, 10, 16, 8, 19, 6, 18, 13, 17, 5, 23, 10, 18, 9, 16, 8, 27, 7, 15, 13, 16, 13, 29, 9, 18, 13, 27, 9, 26, 10, 19, 18, 17, 11, 29, 11, 23, 18, 22, 11, 32 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,4 COMMENTS Number of primes p < n^2 such that p == 1 (mod n). The standard conjecture here is that a(n) ~ n^2/(2 phi(n)log n), where Euler's totient function phi(n) = A000010(n). - Thomas Ordowski, Oct 21 2014 Number of primes appearing in the 1st column of an n X n square array whose elements are the numbers from 1..n^2, listed in increasing order by rows. - Wesley Ivan Hurt, May 17 2021 LINKS Table of n, a(n) for n=1..84. FORMULA a(n) = Sum_{k=1..n} pi(1+n*(k-1)) - pi(n*(k-1)), where pi is the prime counting function. - Wesley Ivan Hurt, May 17 2021 EXAMPLE For n=10, i=1,3,4,6,7 give primes, so a(10)=5. MATHEMATICA f[n_] := Length[ Select[ Range[n - 1], PrimeQ[n# + 1] & ]]; Table[ f[n], {n, 1, 85}] Table[Count[Range[n-1]n+1, _?PrimeQ], {n, 90}] (* Harvey P. Dale, Oct 10 2013 *) 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))) CROSSREFS Cf. A000010 (phi), A000720 (pi). Sequence in context: A001176 A136693 A348045 * A343998 A300586 A094571 Adjacent sequences: A086682 A086683 A086684 * A086686 A086687 A086688 KEYWORD nonn AUTHOR Jon Perry, Jul 28 2003 EXTENSIONS Extended by Robert G. Wilson v, Jul 31 2003 STATUS approved

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Last modified June 10 16:48 EDT 2023. Contains 363205 sequences. (Running on oeis4.)