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A225940 Numbers n such that prime(n) divides prime(2*n) + prime(3*n). 0
1, 4, 67, 84, 88 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Almost surely the sequence is finite and complete, since the ratio (p(3n)+p(2n))/p(n) tends to 5 from above. For n = 5*10^9 the ratio is 5.19958, for n = 10^100 we can estimate it as 5.02. - Giovanni Resta, May 22 2013

LINKS

Table of n, a(n) for n=1..5.

EXAMPLE

Prime(4)=7 divides prime(8)+prime(12)=19+37=56, so 4 is in the sequence.

MATHEMATICA

Select[Range[100], Divisible[Prime[2#]+Prime[3#], Prime[#]]&] (* Harvey P. Dale, Feb 26 2017 *)

PROG

(C)

#include <stdio.h>

#define TOP (1ULL<<32)

typedef unsigned long long U64;

int main() {

  U64 i, j, k, n=1, *primes = (U64*)malloc(TOP);

  char *c = (char*)malloc(TOP/2);

  memset(c, 0, TOP/2);

  for (primes[0] = 2, i = 3; i < TOP; i+=2)

    if (c[i>>1]==0) {

      primes[n++] = i;

      if ((n%3)==0 && (i+primes[n*2/3-1]) % primes[n/3-1]==0)

        printf("%llu, ", n/3);

      for (j = i*i>>1; j < TOP/2; j += i)  c[j] = 1;

    }

  return 0;

}

CROSSREFS

Cf. A000040, A066896.

Sequence in context: A198893 A279886 A048828 * A003360 A225772 A329478

Adjacent sequences:  A225937 A225938 A225939 * A225941 A225942 A225943

KEYWORD

nonn,more

AUTHOR

Alex Ratushnyak, May 21 2013

STATUS

approved

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Last modified April 7 11:07 EDT 2020. Contains 333301 sequences. (Running on oeis4.)