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A001013 Jordan-Polya numbers: products of factorial numbers A000142.
(Formerly M0993 N0372)
22
1, 2, 4, 6, 8, 12, 16, 24, 32, 36, 48, 64, 72, 96, 120, 128, 144, 192, 216, 240, 256, 288, 384, 432, 480, 512, 576, 720, 768, 864, 960, 1024, 1152, 1296, 1440, 1536, 1728, 1920, 2048, 2304, 2592, 2880, 3072, 3456, 3840, 4096, 4320, 4608, 5040, 5184, 5760 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Possible orders of automorphism groups of trees.

Except for the numbers 2, 9 and 10 this sequence is conjectured to be the same as A034878.

Equivalently, a(n)/6*(6x^2-6x+(6x-3)a(n)+2a(n)^2+1) = N^2 has an integer solution. - Ralf Stephan, Dec 04 2004

REFERENCES

R. K. Guy, Unsolved Problems in Number Theory, B23.

Melter, Robert A.; Autometrized unary algebras. J. Combinatorial Theory 5 1968 21-29.

N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).

N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).

LINKS

T. D. Noe, Table of n, a(n) for n=1..987

Eric Weisstein's World of Mathematics, Factorial Products

Index entries for sequences related to trees

EXAMPLE

864=(3!)^2*4!

MATHEMATICA

For[p=0; a=f=Table[n!, {n, 1, 8}], p=!=a, p=a; a=Select[Union@@Outer[Times, f, a], #<=8!&]]; a

CROSSREFS

Cf. A034878.

Sequence in context: A096850 A062847 * A115746 A025610 A131117 A181821

Adjacent sequences:  A001010 A001011 A001012 * A001014 A001015 A001016

KEYWORD

nonn,nice,easy

AUTHOR

N. J. A. Sloane.

EXTENSIONS

More terms, formula from Christian G. Bower, Dec 15 1999.

Edited by Dean Hickerson (dean.hickerson(AT)yahoo.com), Sep 17 2002

STATUS

approved

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Last modified April 19 17:29 EDT 2014. Contains 240767 sequences.