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A250294
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Primes p such that p#-1 is a semiprime, where # is the primorial (A034386).
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1
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7, 17, 29, 31, 43, 59, 71, 73, 97, 101, 223, 233, 257, 439, 503, 709, 859, 863, 1013
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OFFSET
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1,1
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COMMENTS
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a(20) >= 1091. 1091# - 1 is a 458-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(2) = 17 so 17# - 1 = 510509 = 61 * 8369 is a semiprime.
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MATHEMATICA
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p = Select[Range[101], PrimeQ]; p[[ Position[FoldList[Times, p] - 1, _?(PrimeOmega[#] == 2 &)] //Flatten ]] (* Amiram Eldar, Feb 16 2020 *)
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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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a(15)-a(18) using factordb.com from Amiram Eldar, Feb 16 2020
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STATUS
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approved
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