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 A099079 Numbers n such that phi(n).phi(n-1) ... phi(2).phi(1) is prime(dot between numbers means concatenation). 3
 2, 3, 9, 28, 30, 31, 51, 127, 208 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS 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. LINKS C. Rivera, Primes by Listing, The Prime Puzzles & Problems connection. Eric Weisstein's World of Mathematics, Integer Sequence Primes EXAMPLE 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. MATHEMATICA 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 *) CROSSREFS Cf. A046035, A099077, A099078, A099080. Sequence in context: A057296 A057248 A015960 * A006797 A144239 A320951 Adjacent sequences:  A099076 A099077 A099078 * A099080 A099081 A099082 KEYWORD base,more,nonn AUTHOR Farideh Firoozbakht, Oct 23 2004 STATUS approved

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Last modified August 8 11:31 EDT 2020. Contains 336298 sequences. (Running on oeis4.)