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A090502 Numbers n such that tau(n) = tau(n-1)+tau(n+1). 4
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 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

LINKS

Vincenzo Librandi, Table of n, a(n) for n = 1..4000

Vaclav Kotesovec, Plot of a(n)/n for n = 1..1000000

Vaclav Kotesovec, Plot of a(n)/(n*log(log(n))) for n = 1..1000000

MAPLE

with(numtheory); A090502:=proc(i) local n;

for n from 1 to i do

if tau(n)=tau(n+1)+tau(n-1) then print(n); fi;

od; end: A090502 (10^9); # Paolo P. Lava, Apr 24 2013

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: A308324 A044970 A316291 * A324649 A324643 A119425

Adjacent sequences:  A090499 A090500 A090501 * A090503 A090504 A090505

KEYWORD

nonn

AUTHOR

Amarnath Murthy, Dec 05 2003

EXTENSIONS

More terms from Ray Chandler, Dec 09 2003

STATUS

approved

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Last modified April 9 16:39 EDT 2020. Contains 333361 sequences. (Running on oeis4.)