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A250293
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Primes p such that p#+1 is a semiprime, where # is the primorial (A034386).
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2
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13, 19, 23, 43, 61, 67, 73, 83, 101, 139, 151, 173, 223, 251, 383, 457, 571, 673, 761
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OFFSET
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1,1
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COMMENTS
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The next candidate after 571 is 859. 859# + 1 is a 359-digit composite with no known factors. - Hugo Pfoertner, Feb 05 2021
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LINKS
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FORMULA
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EXAMPLE
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a(1) = 13 so 13# + 1 = 30031 = 59 * 509 is a semiprime.
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MATHEMATICA
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Prime[#]&/@(Module[{pmrl=FoldList[Times, Prime[Range[50]]]}, Position[ pmrl, _?(PrimeOmega[ #+1]==2&)]]//Flatten) (* Harvey P. Dale, Apr 29 2017 *)
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CROSSREFS
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KEYWORD
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nonn,hard,more
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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