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A099080
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Numbers n such that sigma(n).sigma(n-1) ... sigma(2).sigma(1) is prime (dot between numbers means 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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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
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LINKS
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Table of n, a(n) for n=1..4.
C. Rivera, Primes by Listing, The Prime Puzzles & Problems connection.
Eric Weisstein's World of Mathematics, Integer Sequence Primes
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EXAMPLE
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3 is in the sequence because sigma(3).sigma(2).sigma(1) = 431 is prime.
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MATHEMATICA
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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 *)
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PROG
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(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
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CROSSREFS
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Cf. A046035, A099077, A099078, A099079.
Sequence in context: A356785 A015169 A041953 * A132532 A108023 A352163
Adjacent sequences: A099077 A099078 A099079 * A099081 A099082 A099083
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KEYWORD
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base,more,nonn
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AUTHOR
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Farideh Firoozbakht, Oct 23 2004
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STATUS
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approved
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