A090886 -id:A090886 - cp's OEIS frontend

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A087400 Primes p such that p+2 is a piptorial number. Also numbers such that A080696(n)- 2 is prime.

Original entry on oeis.org

13, 163, 2803, 3565153, 210344143, 86915972211813115391953, 4419764102942908730796303703, 114681479899746991802547357477494803

Offset: 1

Cino Hilliard, Oct 21 2003

nonn

Piptorial numbers are the partial products of prime-indexed primes.
Sum of reciprocals = 0.08341509210884323904648676616...
a(9) = A080696(1111) - 2 = 1.0954...*10^4885. - Amiram Eldar, Jul 05 2024

			(Product of first four pips) - 2 = 3*5*11*17 - 2 = 2805 - 2 = 2803, which is prime, so 2803 is in the sequence.

Cf. A080696, A090886.

seq[kmax_] := Module[{r = 1, p = 1, s = {}}, Do[p = NextPrime[p]; r *= Prime[p]; If[PrimeQ[r - 2], AppendTo[s, r - 2]], {k, 1, kmax}]; s]; seq[20] (* Amiram Eldar, Jul 05 2024 *)

piptorial(n) = { s=0; p=1; for(x=1,n, p=p*prime(prime(x)); if(isprime(p-2),print1(p-2","); s+=1.0/(p-2)) ); print(); print(s) }

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