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
Offset: 1
Keywords
Examples
4920=211^2-199^2=251^2-241^2=617^2-613^2=1231^2-1229^2
Programs
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C
#define NMAX 200000000 #include
#include #include 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
Extensions
More terms from R. J. Mathar, Oct 05 2006