cp's OEIS Frontend

This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.

A129471 Primes p of Erdos-Selfridge class 3+ with largest prime factor of p+1 not of class 2+.

Original entry on oeis.org

883, 1747, 2417, 2621, 3301, 3533, 3571, 3691, 3853, 4027, 4133, 4783, 4861, 5303, 5381, 5393, 5563, 5641, 5821, 6577, 6991, 7253, 7331, 8059, 8093, 8377, 8839, 8929, 8969, 9221, 9281, 9613, 9931, 10069, 10477, 10487, 10601, 10607, 10903, 11491
Offset: 1

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Author

M. F. Hasler, Apr 17 2007

Keywords

Comments

Examples

			a(1) = 883 = -1+2*13*17 is a prime of class 3+ since 13 is of class 2+, but the largest divisor of 883+1 is 17 which is only of class 1+.
		

Crossrefs

Programs

  • PARI
    class(n,s=1)={n=factor(n+s)[,1];if(n[ #n]<=3,1,for(i=2,#n,n[1]=max(class(n[i],s)+1,n[1]));n[1])}; A129471(n=100,p=1,a=[])={ local(f); while( #a 3 & 2 > class(f[ #f]), f=factor(1+p=nextprime(p+1))[,1]); forstep( i=#f-1,2,-1, if( 3 < f[1] = max( f[1],1+class( f[i] )), next(2))); if( f[1] == 3, a=concat(a,p); /*print(#a," ",p)*/)); a}