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.

1, 7, 232, 430

Offset: 1

Author

Ralf Stephan, Mar 24 2003

Keywords

• nonn
• hard
• more

Comments

a(5) is greater than 10^7. - Matty Muir, Oct 01 2020

Examples

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

Crossrefs

Cf. A066735.

Programs

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 *)

PARI

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