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A081618 Numbers n such that (product of first n primes)+1 is divisible by the (n+1)-th prime. Also n such that A075306(n)-1 is equal to A002110(n). Positions of 1 in A081617. 2
1, 7, 232, 430 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

a(5) is greater than 5*10^5. - Matty Muir, Nov 28 2019

LINKS

Table of n, a(n) for n=1..4.

EXAMPLE

The 8th prime, 19, divides 2*3*5*7*11*13*17+1=510511, thus 7 is a member.

MATHEMATICA

With[{nn=500}, Flatten[Position[Thread[{Rest[FoldList[Times, 1, Prime[ Range[ nn]]]]+ 1, Prime[ Range[2, nn+1]]}], _?(Divisible[#[[1]], #[[2]]]&), {1}, Heads->False]]] (* Harvey P. Dale, Apr 18 2015 *)

PROG

(PARI) p=1; for(n=1, 10^5, p=p*prime(n); if((p+1)%prime(n+1)==0, print1(n", ")))

CROSSREFS

Cf. A066735.

Sequence in context: A221495 A142116 A201955 * A285369 A287042 A160491

Adjacent sequences:  A081615 A081616 A081617 * A081619 A081620 A081621

KEYWORD

nonn,hard,more

AUTHOR

Ralf Stephan, Mar 24 2003

STATUS

approved

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Last modified July 4 09:40 EDT 2020. Contains 335446 sequences. (Running on oeis4.)