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Search: id:A074240
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| A074240 |
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Numbers n such that n-th prime = sigma(sigma(n)) + 1. |
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+0
1
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OFFSET
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1,2
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COMMENT
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There are no other numbers < 3400000. - Stefan Steinerberger (hansibal(AT)hotmail.com), Nov 20 2005
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EXAMPLE
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sigma(sigma(110)) + 1 = sigma(216) + 1 = 601 = prime(110).
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MATHEMATICA
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Select[Range[10^6], Prime[ # ] == DivisorSigma[1, DivisorSigma[1, # ]] + 1 &]
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PROGRAM
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(MuPAD): ind := 1; nu := 2; repeat w := numlib::sigma(numlib::sigma(ind))+1; if w = nu then print(ind); end_if; ind := ind+1; nu := nextprime(nu+1); until ind>3400000 end_repeat; (Steinerberger)
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CROSSREFS
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Sequence in context: A134213 A028995 A063668 this_sequence A008450 A035836 A008395
Adjacent sequences: A074237 A074238 A074239 this_sequence A074241 A074242 A074243
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KEYWORD
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nonn
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AUTHOR
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Joseph L. Pe (joseph_l_pe(AT)hotmail.com), Sep 18 2002
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