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A099079
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Numbers n such that phi(n).phi(n-1). ... .phi(2).phi(1) is prime (dots between numbers mean concatenation).
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3
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OFFSET
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1,1
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COMMENTS
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Number of digits of primes corresponding to the nine known terms of this sequence are respectively 2,3,9,39,42,44,84,244,441.
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LINKS
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EXAMPLE
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9 is in the sequence because phi(9).phi(8).phi(7).phi(6).phi(5).phi(4).phi(3).phi(2).phi(1) = 646242211 is prime.
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MATHEMATICA
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Module[{nn=210, eph}, eph=EulerPhi[Range[nn]]; Position[Table[FromDigits[ Flatten[ IntegerDigits[Reverse[Take[eph, n]]]]], {n, nn}], _?PrimeQ]]// Flatten (* Harvey P. Dale, Apr 21 2020 *)
ParallelTable[If[PrimeQ[ToExpression[StringJoin[ToString[#]&/@Reverse[Table[EulerPhi[k], {k, 1, n}]]]]], n, Nothing], {n, 1, 10^4}]//.{}->Nothing (* J.W.L. (Jan) Eerland, Aug 15 2022 *)
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CROSSREFS
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KEYWORD
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base,more,nonn,changed
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AUTHOR
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STATUS
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approved
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