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A137518 a(1)=2. For n>=2, a(n) = smallest integer > a(n-1) that has the same number of divisors as n has. 4
2, 3, 5, 9, 11, 14, 17, 21, 25, 26, 29, 32, 37, 38, 39, 81, 83, 92, 97, 98, 106, 111, 113, 114, 121, 122, 123, 124, 127, 128, 131, 147, 155, 158, 159, 196, 197, 201, 202, 222, 223, 230, 233, 236, 242, 247, 251, 272, 289, 292, 295, 316, 317, 318, 319, 322, 323 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Starting the sequence at a(1)=1 instead would lead to a(n) = n for all positive integers n.

LINKS

Harvey P. Dale, Table of n, a(n) for n = 1..1000

MATHEMATICA

a = {2}; Do[i = a[[ -1]] + 1; While[ ! Length[Divisors[i]] == Length[Divisors[n]], i++ ]; AppendTo[a, i], {n, 2, 80}]; a (* Stefan Steinerberger, Apr 26 2008 *)

nxt[{a_, n_}]:=Module[{k=a+1, div=DivisorSigma[0, n+1]}, While[div != DivisorSigma[ 0, k], k++]; {k, n+1}]; Transpose[NestList[nxt, {2, 1}, 60]] [[1]] (* Harvey P. Dale, May 28 2014 *)

CROSSREFS

Sequence in context: A215779 A140329 A163292 * A137509 A014109 A102940

Adjacent sequences:  A137515 A137516 A137517 * A137519 A137520 A137521

KEYWORD

nonn

AUTHOR

Leroy Quet, Apr 24 2008

EXTENSIONS

More terms from Stefan Steinerberger, Apr 26 2008

STATUS

approved

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Last modified December 15 14:37 EST 2019. Contains 329999 sequences. (Running on oeis4.)