In mathematics, the bimonster is a group that is the wreath product of the monster group M with Z₂:

${\displaystyle Bi=M\wr \mathbb {Z} _{2}.\,}$

The Bimonster is also a quotient of the Coxeter group corresponding to the Dynkin diagram Y₅₅₅, a Y-shaped graph with 16 nodes:

Actually, the 3 outermost nodes are redundant. This is because the subgroup Y₁₂₄ is the E₈ Coxeter group. It generates the remaining node of Y₁₂₅. This pattern extends all the way to Y₄₄₄: it automatically generates the 3 extra nodes of Y₅₅₅.

John H. Conway conjectured that a presentation of the bimonster could be given by adding a certain extra relation to the presentation defined by the Y₄₄₄ diagram. More specifically, the affine E6 Coxeter group is ${\displaystyle \mathbb {Z} ^{6}:O_{5}(3):2}$, which can be reduced to the finite group ${\displaystyle 3^{5}:O_{5}(3):2}$ by adding a single relation called the spider relation. Once this relation is added, and the diagram is extended to Y₄₄₄, the group generated is the bimonster. This was proved in 1990 by Simon P. Norton; the proof was simplified in 1999 by A. A. Ivanov.

Many subgroups of the (bi)monster can be defined by adjoining the spider relation to smaller Coxeter diagrams, most notably the Fischer groups and the baby monster group. The groups Y_ij0, Y_ij1, Y₁₂₂, Y₁₂₃, and Y₁₂₄ are finite even without adjoining additional relations. They are the Coxeter groups A_i+j+1, D_i+j, E₆, E₇, and E₈, respectively. Other groups, which would be infinite without the spider relation, are summarized below:

Y-group name	Group generated
Y222	${\displaystyle 3^{5}:O_{5}(3):2}$
Y223	${\displaystyle O_{7}(3)\times 2}$
Y224	${\displaystyle O_{8}^{+}(3):2}$[note 1]
Y133	${\displaystyle 2^{7}:(O_{7}(2)\times 2)}$[note 2]
Y134	${\displaystyle O_{9}(2)\times 2}$[note 2]
Y144	${\displaystyle O_{10}^{-}(2):2}$[note 2]
Y233	${\displaystyle 2\times 2Fi_{22}}$
Y234	${\displaystyle 2\times Fi_{23}}$
Y244	${\displaystyle 3.Fi_{24}}$
Y333	${\displaystyle 2\times 2^{2}.^{2}E_{6}(2)}$
Y334	${\displaystyle 2\times 2.\mathbb {B} }$
Y344	${\displaystyle 2\times \mathbb {M} }$
Y444	${\displaystyle \mathbb {M} \wr 2}$[note 3]

• Triality - simple Lie group D₄, Y₁₁₁
• Affine E_6 Y₂₂₂

• Basak, Tathagata (2007), "The complex Lorentzian Leech lattice and the Bimonster", Journal of Algebra, 309 (1): 32–56, arXiv:math/0508228, doi:10.1016/j.jalgebra.2006.05.033, MR 2301231, S2CID 125231322.
• Ivanov, A. A. (1999), "Y-groups via Transitive Extension", Journal of Algebra, 218 (1): 142–435, doi:10.1006/jabr.1999.7882.
• Conway, John H.; Norton, Simon P.; Soicher, Leonard H. (1988), "The Bimonster, the group Y₅₅₅, and the projective plane of order 3", Computers in Algebra (Chicago, IL, 1985), Lecture Notes in Pure and Applied Mathematics, vol. 111, New York: Dekker, pp. 27–50, MR 1060755.
• Conway, J. H.; Pritchard, A. D. (1992), "Hyperbolic reflections for the Bimonster and 3Fi₂₄", Groups, Combinatorics & Geometry (Durham, 1990), London Math. Soc. Lecture Note Ser., vol. 165, Cambridge: Cambridge University Press, pp. 24–45, doi:10.1017/CBO9780511629259.006, MR 1200248.
• Conway, John H.; Simons, Christopher S. (2001), "26 implies the Bimonster", Journal of Algebra, 235 (2): 805–814, doi:10.1006/jabr.2000.8494, MR 1805481.
• Simons, Christopher Smyth (1997), Hyperbolic reflection groups, completely replicable functions, the Monster and the Bimonster, Ph.D. thesis, Princeton University, Department of Mathematics, ISBN 978-0591-50546-7, MR 2696217.
• Soicher, Leonard H. (1989), "From the Monster to the Bimonster", Journal of Algebra, 121 (2): 275–280, doi:10.1016/0021-8693(89)90064-1, MR 0992763.

• Weisstein, Eric W. "Bimonster". MathWorld. (Note: incorrectly named here as [3^6,6,6])

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