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A090502
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Numbers k such that tau(k) = tau(k-1) + tau(k+1).
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5
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6, 20, 28, 32, 52, 56, 68, 152, 174, 176, 184, 186, 208, 212, 248, 266, 268, 272, 292, 322, 328, 338, 464, 470, 472, 476, 518, 534, 536, 578, 582, 585, 590, 602, 606, 650, 656, 658, 666, 670, 686, 710, 754, 772, 788, 790, 804, 814, 836, 846, 848, 894, 988
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OFFSET
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1,1
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LINKS
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MATHEMATICA
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Select[Range[1000], DivisorSigma[0, #]==DivisorSigma[0, #-1]+ DivisorSigma[ 0, #+1]&] (* Harvey P. Dale, Feb 27 2012 *)
Flatten[Position[Partition[DivisorSigma[0, Range[1000]], 3, 1], _?(#[[2]]==#[[1]]+#[[3]]&), 1, Heads->False]]+1 (* Much faster than the above Mathematica program because tau of each number only has to be calculated once. *) (* Harvey P. Dale, Jan 03 2020 *)
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PROG
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CROSSREFS
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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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