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A009287 a(1) = 3; thereafter a(n+1) = least k with a(n) divisors. 8
3, 4, 6, 12, 60, 5040, 293318625600, 670059168204585168371476438927421112933837297640990904154667968000000000000 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

The sequence must start with 3, since a(1)=1 or a(1)=2 would lead to a constant sequence. - M. F. Hasler, Sep 02 2008

The calculation of a(7) and a(8) is based upon the method in A037019 (which, apparently, is the method previously used by the authors of A009287). So a(7) and a(8) are correct unless n=a(6)=5040 or n=a(7)=293318625600 are "exceptional" as described in A037019. - Rick L. Shepherd, Aug 17 2006

a(7) is correct because 5040 not exceptional (see A072066). - T. D. Noe, Sep 02 2008

Terms from a(2) to a(7) are highly composite (that is, found in A002182), but a(8) is not. - Ivan Neretin, Mar 28 2015 [Equivalently, the first 6 terms are in A002183, but a(7) is not. Note that the smallest number with at least a(7) divisors is A002182(695) ~ 1.77 * 10^59 with 293534171136 divisors, which is much smaller than a(8) ~ 6.70 * 10^75. - Jianing Song, Jul 15 2021]

Grime reported that Ramanujan unfortunately missed a(7) with 5040 divisors. - Frank Ellermann, Mar 12 2020

REFERENCES

Amarnath Murthy, Pouring a few more drops in the ocean of Smarandache Sequences and Conjectures (to be published in the Smarandache Notions Journal) [Note: this author submitted two erroneous versions of this sequence to the OEIS, A036480 and A061080, entries which contained invalid conjectures.]

LINKS

Table of n, a(n) for n=1..8.

Jason Earls, A note on the Smarandache divisors of divisors sequence and two similar sequences, in Smarandache Notions Journal (2004), Vol. 14.1, page 274.

James Grime and Brady Haran, Infinite Anti-Primes, Numberphile video (2016).

FORMULA

a(n) = A005179(a(n-1)).

EXAMPLE

5040 is the smallest number with 60 divisors.

MATHEMATICA

f[n_] := Block[{k = 3, s = (Times @@ (Prime[Range[Length@ #]]^Reverse[# - 1])) & @ Flatten[FactorInteger[#] /. {a_Integer, b_} :> Table[a, {b}]] & /@ Range@ 10000}, Reap@ Do[Sow[k = s[[k]]], {n}] // Flatten // Rest]; f@ 6 (* Michael De Vlieger, Mar 28 2015, after Wouter Meeussen at A037019 *)

CROSSREFS

Cf. A000005, A005179, A037019.

Sequence in context: A202855 A182857 A251483 * A061080 A254049 A280289

Adjacent sequences:  A009284 A009285 A009286 * A009288 A009289 A009290

KEYWORD

nonn

AUTHOR

David W. Wilson and James Kilfiger (jamesk(AT)maths.warwick.ac.uk)

EXTENSIONS

Entry revised by N. J. A. Sloane, Aug 25 2006

STATUS

approved

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Last modified June 30 11:55 EDT 2022. Contains 354939 sequences. (Running on oeis4.)