The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation. Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A336016 a(n) is the number of primes q less than primorial(n) having k = 2 as the least exponent such that q^k == 1 (mod primorial(n)). 2
 0, 1, 3, 5, 8, 19, 22, 51, 89, 145, 263, 453, 851, 1575, 2880, 5469, 10338, 19115, 35782, 67569, 128601, 243600, 463840, 883589 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS a(n) = length of row n of A336015 for n > 1. LINKS EXAMPLE a(4) = 5 as there are 5 primes q coprime to primorial(4) = 210 such that 2 is the least positive integer exponent k where q^k == 1 (mod 210). Those primes are 29, 41, 71, 139, 181 and indeed we have 29^2 == 1 (mod 210), 41^2 == 1 (mod 210), 71^2 == 1 (mod 210), 139^2 == 1 (mod 210) and 181^2 == 1 (mod 210) and no more below 210. So as these are five such primes in row 4, a(4) = 5. - David A. Corneth, Aug 15 2020 MATHEMATICA Table[Block[{P = #, k = 0}, Do[If[MultiplicativeOrder[Prime@ i, P] == 2, k++], {i, PrimePi[n + 1], PrimePi[P - 1]}]; k] &@ Product[Prime@ j, {j, n}], {n, 8}] PROG (PARI) a(n) = {if(n <= 2, return(n-1)); my(pp = vecprod(primes(n))/2, d = divisors(pp), res = 0); for(i = 1, #d, c = lift(chinese(Mod(-1, d[i]), Mod(1, pp/d[i]))); forstep(i = c, pp*2, pp, if(isprime(i), res++ ) ) ); res } \\ David A. Corneth, Aug 16 2020 CROSSREFS Cf. A000010, A002110, A005867, A131577, A336015. Sequence in context: A240532 A184434 A196140 * A240154 A112656 A002366 Adjacent sequences:  A336013 A336014 A336015 * A336017 A336018 A336019 KEYWORD nonn,more AUTHOR Michael De Vlieger, David James Sycamore, David A. Corneth, Jul 10 2020 EXTENSIONS New name from David A. Corneth, Aug 15 2020 STATUS approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified May 12 03:08 EDT 2021. Contains 343810 sequences. (Running on oeis4.)