A091878 - OEIS

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A091878

a(n) can be expressed as the difference of the squares of consecutive primes in just four distinct ways.

4920, 801528, 1484280, 5351640, 7257720, 7647360, 12168912, 12302472, 15540360, 17348520, 21623160, 25639320, 27285048, 27462840, 27470352, 34714680, 35684040, 38466288, 48449640, 49936152, 51272760, 54037368, 60948888

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OFFSET

1,1

LINKS

Table of n, a(n) for n=1..23.

EXAMPLE

4920=211^2-199^2=251^2-241^2=617^2-613^2=1231^2-1229^2

PROG

(C) #define NMAX 200000000 #include <stdlib.h> #include <stdio.h> #include <limits.h> int isprime(const int n) { for(int i=2 ; i*i <= n ; i++) if( n % i == 0) return(0) ; return 1 ; } int main(int argc, char *argv[]) { short * n= (short*)calloc(NMAX, sizeof(short)) ; int wm=0; for(int p=2 ; ; ) { int np=p+1 ; while( !isprime(np) ) np++ ; if(np<0) break ; if ( p+np < INT_MAX/(np-p) ) { const int i=(p+np)*(np-p) ; const int nw= p+np ; if( i < NMAX ) n[i]++ ; for(int j=wm ; j < nw ; j++) if ( n[j] == 4) printf("%d ", j) ; wm=nw ; } p=np ; if( p > INT_MAX-np ) break ; } free(n) ; } - R. J. Mathar, Oct 05 2006

CROSSREFS

Cf. A078667, A090783, A090784, A090785.

Sequence in context: A255983 A043480 A028550 * A015357 A241934 A185850

Adjacent sequences: A091875 A091876 A091877 * A091879 A091880 A091881

KEYWORD

nonn

AUTHOR

Ray G. Opao and Ray Chandler, Feb 11 2004

EXTENSIONS

More terms from R. J. Mathar, Oct 05 2006

STATUS

approved