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A103538
Numbers k such that prime(k)*prime(k)# - 1 is prime, where prime(k)# is the k-th primorial.
0
1, 2, 3, 5, 6, 7, 13, 16, 20, 23, 64, 191, 490, 3620, 5909
OFFSET
1,2
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
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