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A109240
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Numbers n such that the concatenation of consecutive increasing numbers beginning with prime(n) and ending with prime(n+1) is semiprime; or n such that A111875(n) is semiprime.
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0
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12, 16, 31, 55, 74, 84, 86, 93, 108, 114, 133, 161, 164, 170, 200, 211, 218, 224, 231, 242, 252, 271, 287, 301, 347, 358, 449, 450, 505, 520, 556, 576, 597, 621, 622, 629, 632, 657, 667, 703, 716, 733, 756, 767, 782, 788, 789, 813, 814, 834, 850, 852, 882
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OFFSET
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1,1
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LINKS
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EXAMPLE
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a(3)=31 because prime(31)=127 and prime(32)=131 and 127128129130131 =
3*42376043043377.
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MATHEMATICA
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Select[Range[900], PrimeOmega[FromDigits[Flatten[IntegerDigits/@ Range[ Prime[ #], Prime[ #+1]]]]] == 2&] (* Harvey P. Dale, Jun 16 2021 *)
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CROSSREFS
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KEYWORD
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less,nonn,base
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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