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A129478 Primes p of Erdos-Selfridge class 7+ with largest prime factor of p+1 not of class 6+. 6
17227801, 18207913, 18592957, 19433053, 19608073, 19678081, 20028121, 20518177, 20658193, 20833213, 21043237, 21218257, 21533293, 21743317, 22128361, 22303381, 23668537, 25068697, 25418737, 25453741 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

See A129470

LINKS

Table of n, a(n) for n=1..20.

EXAMPLE

a(1) = 17227801 = -1+2*2917*2953 = A129469[7] is a prime of class 7+ since 17227801+1 has prime factor 2917 = A081634[1] = A005113[6] of class 6+, but the largest prime factor of 17227801+1 is 2953 = A005107[175] of class 3+.

PROG

(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])}; a129478(n=100, p=1, a=[])={local(f, a6=A005113[6]); p=max(p, a6*nextprime(a6+1)*2-2); while( #a<n, until( #f>2 & f[ #f-1] >= a6 & 6 > class(f[ #f]), f=factor(1+p=nextprime(p+1))[, 1]); forstep( i=#f-1, 2, -1, if( 7 < f[1] = max( f[1], 1+class( f[i] )), next(2))); if( f[1] == 7, a=concat(a, p); print(#a, " ", p))); a}

CROSSREFS

Subsequence of A129470; see also A129471-A129473, A129477, A129469, A005113, A005105-A005108, A081633-A081635.

Sequence in context: A172597 A172569 A254000 * A326137 A288078 A183269

Adjacent sequences:  A129475 A129476 A129477 * A129479 A129480 A129481

KEYWORD

nonn

AUTHOR

M. F. Hasler, Apr 17 2007

STATUS

approved

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Last modified June 20 17:27 EDT 2019. Contains 324234 sequences. (Running on oeis4.)