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A190821
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Prime numbers p where d(p-1) = d(p+1) increases to a record.
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0
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7, 19, 41, 199, 919, 5741, 18089, 41651, 90271, 446081, 1276001, 27033161, 43220449, 53308529, 109245401, 512669249, 663929729, 2266639649, 2560742911, 2969200961, 8505402751, 32540473601, 61573368401, 74335064959, 109494811999
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OFFSET
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1,1
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COMMENTS
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LINKS
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EXAMPLE
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a(1) = 7 because 7 is prime and d(6) = 4 = d(8).
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MATHEMATICA
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s = Select[Prime@ Range@ 1000000, DivisorSigma[0, # - 1] == DivisorSigma[0, # + 1] &]; t = DivisorSigma[0, # - 1] & /@ s; a = {0}; b = {0}; Do[If[t[[k]] > Max@ b, AppendTo[a, s[[k]]]]; AppendTo[b, t[[k]]], {k, Length@ s}]; a (* Michael De Vlieger, Oct 30 2015 *)
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PROG
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(PARI) r=0; forprime(p=2, 4e9, t=numdiv(p-1); if(t>r&t==numdiv(p+1), r=t; print1(p", "))) \\ Charles R Greathouse IV, May 27 2011
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CROSSREFS
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Cf. A145337, A190646 (numbers n such that d(n-1)=d(n+1) increases to a record).
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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