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A213407 First occurrence of n in A213381. 2
2, 0, 3, 5, 4, 9, 8, 7, 12, 17, 594270126787, 21, 20, 13, 24, 29, 16, 23, 32, 19, 36, 41, 1356, 45, 38, 151, 51, 411, 994, 57, 56, 31, 60, 65, 52, 69, 71, 37, 72, 77, 490, 81, 80, 169, 84, 6125, 68, 1043, 92, 49, 99, 101, 304, 105, 104, 55, 111, 63, 73, 89, 116, 61, 120, 125, 78, 129, 188, 67, 93, 137, 97, 18349355, 140, 433 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,1

LINKS

Table of n, a(n) for n=0..73.

FORMULA

a(n) = smallest k>n such that (-2)^k == n (mod k+2).

EXAMPLE

Smallest n such that A213381(n)=9 is 17, so a(9)=17.

PROG

(C)

#include <stdio.h> // GCC -O3 -fopenmp

#include <omp.h>  // 11 minutes

#define SIZE 4096

#define STEP 0x1000000

long long first[SIZE];

int main(int argc, char **argv)

{

    unsigned long long a, i;

    for (a=0; a<SIZE; ++a)  first[a]=-1LL;

    omp_set_num_threads(4);

    for (i=STEP; i<=0x100000000; i+=STEP) {

      #pragma omp parallel for schedule(static, STEP/4)

      for (a=i-STEP; a<i; ++a) {

        unsigned long long b, r, p, t;

        for (b=a+2, r=1, p=t=a; t>0; p=(p*p)%b, t>>=1) {

            if (t&1) r=(r*p)%b;

        }

        if (r<SIZE)  { if (a<(unsigned)first[r]) first[r]=a; }

      }

      printf("\n%llx : ", i);

      for (a=0; a<SIZE; ++a)  if (first[a]==-1LL) printf("%llu ", a);

    }

    printf("\n\n");

    for (a=0; a<500; ++a)  printf("%lld, ", first[a]);

}

CROSSREFS

Cf. A213381, A213861

Sequence in context: A196503 A061151 A249774 * A285310 A262946 A343932

Adjacent sequences:  A213404 A213405 A213406 * A213408 A213409 A213410

KEYWORD

nonn

AUTHOR

Alex Ratushnyak, Jun 10 2012

EXTENSIONS

Terms a(10) onward from Max Alekseyev, Jan 31 2014

STATUS

approved

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Last modified November 30 18:40 EST 2021. Contains 349424 sequences. (Running on oeis4.)