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 A099080 Numbers n such that sigma(n).sigma(n-1) ... sigma(2).sigma(1) is prime (dot between numbers means concatenation). 3
 2, 3, 66, 102 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Numbers of digits of primes corresponding to the four known terms of this sequence are respectively 2, 3, 133, and 232. A naive heuristic suggests that this sequence is infinite but extremely sparse. - Charles R Greathouse IV, Nov 05 2013 There are no more terms below 10000. - Charles R Greathouse IV, Nov 09 2013 LINKS C. Rivera, Primes by Listing, The Prime Puzzles & Problems connection. Eric Weisstein's World of Mathematics, Integer Sequence Primes EXAMPLE 3 is in the sequence because sigma(3).sigma(2).sigma(1) = 431 is prime. MATHEMATICA Module[{nn=110, d}, d=DivisorSigma[1, Range[nn]]; Select[Range[nn], PrimeQ[ FromDigits[ Flatten[IntegerDigits/@Reverse[Take[d, #]]]]]&]] (* Harvey P. Dale, Jul 25 2016 *) PROG (PARI) s="1"; for(n=2, 1e3, s=Str(sigma(n), s); if(ispseudoprime(eval(s)), print1(n", "))) \\ Charles R Greathouse IV, Nov 05 2013 CROSSREFS Cf. A046035, A099077, A099078, A099079. Sequence in context: A356785 A015169 A041953 * A132532 A108023 A352163 Adjacent sequences: A099077 A099078 A099079 * A099081 A099082 A099083 KEYWORD base,more,nonn AUTHOR Farideh Firoozbakht, Oct 23 2004 STATUS approved

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Last modified April 1 03:44 EDT 2023. Contains 361673 sequences. (Running on oeis4.)