A250293 Primes p such that p#+1 is a semiprime, where # is the primorial (A034386).
13, 19, 23, 43, 61, 67, 73, 83, 101, 139, 151, 173, 223, 251, 383, 457, 571, 673, 761
Offset: 1
Examples
a(1) = 13 so 13# + 1 = 30031 = 59 * 509 is a semiprime.
Links
- factordb.com, Status of 859#+1.
- Hisanori Mishima, Factorizations of many number sequences, PI Pn + 1 (n = 1 to 110) (2012).
Programs
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Mathematica
Prime[#]&/@(Module[{pmrl=FoldList[Times,Prime[Range[50]]]},Position[ pmrl, ?(PrimeOmega[ #+1]==2&)]]//Flatten) (* _Harvey P. Dale, Apr 29 2017 *)
Formula
a(n) = prime(A085725(n)). - Hugo Pfoertner, Feb 05 2021
Extensions
a(16)-a(18) using factordb.com from Hugo Pfoertner, Feb 05 2021
Missing 571 inserted by Sean A. Irvine, Mar 03 2023
Comments