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A128555 a(n) = the smallest positive multiple of d(n) that does not occur earlier in the sequence, where d(n) is the number of positive divisors of n. 2
1, 2, 4, 3, 6, 8, 10, 12, 9, 16, 14, 18, 20, 24, 28, 5, 22, 30, 26, 36, 32, 40, 34, 48, 15, 44, 52, 42, 38, 56, 46, 54, 60, 64, 68, 27, 50, 72, 76, 80, 58, 88, 62, 66, 78, 84, 70, 90, 21, 96, 92, 102, 74, 104, 100, 112, 108, 116, 82, 120, 86, 124, 114, 7, 128, 136, 94, 126 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

This sequence is a permutation of the positive integers.

LINKS

Ivan Neretin, Table of n, a(n) for n = 1..10000

EXAMPLE

8 has 4 positive divisors. So a(8) is the smallest positive multiple of 4 that has yet to appear in the sequence. 4 and 8 occur among the first 7 terms of the sequence, but 12 does not. So a(8) = 12.

MAPLE

A128555 := proc(nmin) local a, n, d, k ; a := [1, 2] ; while nops(a) < nmin do n := nops(a)+1 ; d := numtheory[tau](n) ; k := 1; while k*d in a do k := k+1 ; od; a := [op(a), k*d] ; od: RETURN(a) ; end: A128555(80) ; # R. J. Mathar, Oct 09 2007

MATHEMATICA

a = {1}; Do[AppendTo[a, Min[Complement[Range[Max[a] + 1]*DivisorSigma[0, n], a]]], {n, 2, 68}]; a (* Ivan Neretin, May 03 2015 *)

CROSSREFS

Cf. A128556.

Sequence in context: A120241 A352714 A155487 * A039874 A193991 A114107

Adjacent sequences:  A128552 A128553 A128554 * A128556 A128557 A128558

KEYWORD

nonn

AUTHOR

Leroy Quet, Mar 10 2007

EXTENSIONS

More terms from R. J. Mathar, Oct 09 2007

STATUS

approved

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Last modified May 24 11:04 EDT 2022. Contains 354033 sequences. (Running on oeis4.)