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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

Table of n, a(n) for n=1..9.

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.)