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A137689 Indices n such that A128646(n)-1 is prime, where A128646 = denominator of partial sums of 1/(p(i)-1). 3
3, 4, 5, 7, 8, 9, 10, 11, 15, 16, 23, 24, 26, 47, 48, 54, 78, 79, 80, 243, 244, 245, 246, 247, 367, 368, 369, 370, 371, 372, 373, 374, 375, 447, 453, 635, 636, 1656, 1657, 1658, 1659, 1660 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Terms corresponding to indices n = a(k) > 1000 are not certified primes but at least probable primes. Is there a simple explanation for the large gaps between a(k)=80, a(k+1)=243 and a(k)=636, a(k+1)=1656 ?

LINKS

Table of n, a(n) for n=1..42.

PROG

(PARI) print_A137689(i=0/*start checking at i+1*/)={local(s=sum(j=1, i, 1/(prime(j)-1))); while(1, while(!ispseudoprime(-1+denominator(s+=1/(prime(i++)-1))), ); print1(i", "))

CROSSREFS

Cf. A128646, A137690-A137692, A092063.

Sequence in context: A278637 A191979 A072225 * A081690 A081688 A118170

Adjacent sequences:  A137686 A137687 A137688 * A137690 A137691 A137692

KEYWORD

hard,more,nonn

AUTHOR

M. F. Hasler, Feb 07 2008

EXTENSIONS

Edited by T. D. Noe, Oct 30 2008

STATUS

approved

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Last modified June 22 04:06 EDT 2021. Contains 345367 sequences. (Running on oeis4.)