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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

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.

FORMULA

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

EXAMPLE

5040 is the smallest number with 60 divisors.

PROG

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 August 21 15:56 EDT 2017. Contains 290890 sequences.