login
The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A202472 Goldbach's Problem extended to subtraction: number of decompositions of 2n into unordered differences of two primes, p, q, where p < 2n < q. 4
0, 1, 1, 2, 2, 3, 2, 3, 3, 3, 2, 6, 4, 3, 6, 3, 4, 6, 4, 5, 8, 4, 4, 7, 6, 4, 9, 8, 4, 11, 5, 5, 11, 6, 8, 9, 4, 7, 11, 7, 4, 13, 7, 5, 15, 7, 8, 13, 8, 9, 11, 7, 7, 13, 10, 5, 13, 7, 7, 19, 9, 8, 17, 9, 10, 16, 9, 9, 15, 12, 7, 19, 9, 7, 19, 9, 12, 17, 8, 14 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,4

LINKS

T. D. Noe, Table of n, a(n) for n = 1..10000

MATHEMATICA

Table[Length[Select[Prime[Range[PrimePi[2*n]]], PrimeQ[2*n + #] &]], {n, 100}] (* T. D. Noe, Apr 16 2013 *)

PROG

(PARI) a(n)=my(s); forprime(p=2, 2*n, s+=isprime(2*n+p)); s \\ Charles R Greathouse IV, Dec 19 2011

(C++)

#include <iostream>

using namespace std;

int main()

{ int p[25] = {2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97};

  int count, istart = 2;

  for(int n=1; n<=25; n++)

  {

      if(2*n>p[istart]) istart++;

      count = 0;

      for(int j=1; p[j]<2*n; j++)

        for(int i=istart; p[i]-p[j]<=2*n; i++)

          if(p[i]-p[j]==2*n) count++;

      cout << n << ". " << count << endl;

  }

    return 0;

} // code for the first 25 integers, James D. Klein, Dec 21 2011

CROSSREFS

Extension of A002375.

Sequence in context: A304333 A136510 A080071 * A235613 A322418 A019569

Adjacent sequences:  A202469 A202470 A202471 * A202473 A202474 A202475

KEYWORD

nonn

AUTHOR

James D. Klein, Dec 19 2011

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.

License Agreements, Terms of Use, Privacy Policy. .

Last modified August 14 07:04 EDT 2020. Contains 336477 sequences. (Running on oeis4.)