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A090502
Numbers k such that tau(k) = tau(k-1) + tau(k+1).
5
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
OFFSET
1,1
MATHEMATICA
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 *)
PROG
(PARI) is(n)=numdiv(n)==numdiv(n-1)+numdiv(n+1) \\ Charles R Greathouse IV, Apr 24 2013
CROSSREFS
Cf. A074757.
Sequence in context: A044970 A345910 A316291 * A324649 A324643 A119425
KEYWORD
nonn
AUTHOR
Amarnath Murthy, Dec 05 2003
EXTENSIONS
More terms from Ray Chandler, Dec 09 2003
STATUS
approved