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A091878 a(n) can be expressed as the difference of the squares of consecutive primes in just four distinct ways. 4
4920, 801528, 1484280, 5351640, 7257720, 7647360, 12168912, 12302472, 15540360, 17348520, 21623160, 25639320, 27285048, 27462840, 27470352, 34714680, 35684040, 38466288, 48449640, 49936152, 51272760, 54037368, 60948888 (list; graph; refs; listen; history; text; internal format)
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

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Last modified October 19 04:40 EDT 2019. Contains 328211 sequences. (Running on oeis4.)