A103538 - OEIS

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A103538

Numbers k such that prime(k)*prime(k)# - 1 is prime, where prime(k)# is the k-th primorial.

1, 2, 3, 5, 6, 7, 13, 16, 20, 23, 64, 191, 490, 3620, 5909

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OFFSET

1,2

LINKS

Table of n, a(n) for n=1..15.

MATHEMATICA

q[n_] := PrimeQ[Prime[n]*Product[Prime[i], {i, 1, n}] - 1]; Select[Range[500], q] (* Amiram Eldar, Jul 18 2021 *)

Module[{nn=500, p1, p2}, p1=Prime[Range[nn]]; p2=FoldList[Times, p1]; Select[Thread[ {p1, p2}], PrimeQ[ Times@@#-1]&][[;; , 1]]]//PrimePi (* Harvey P. Dale, May 21 2023 *)

CROSSREFS

Cf. A002110.

Sequence in context: A088968 A328451 A057924 * A144671 A073721 A285639

Adjacent sequences: A103535 A103536 A103537 * A103539 A103540 A103541

KEYWORD

nonn,more

AUTHOR

Pierre CAMI, Mar 22 2005

EXTENSIONS

a(14) from Amiram Eldar, Jul 18 2021

a(15) from Michael S. Branicky, May 28 2023

STATUS

approved