Co₁ is one of the 26 sporadic groups and was discovered by John Horton Conway in 1968. It is the largest of the three sporadic Conway groups and can be obtained as the quotient of Co₀ (group of automorphisms of the Leech lattice Λ that fix the origin) by its center, which consists of the scalar matrices ±1. It also appears at the top of the automorphism group of the even 26-dimensional unimodular lattice II_25,1. Some rather cryptic comments in Witt's collected works suggest that he found the Leech lattice and possibly the order of its automorphism group in unpublished work in 1940.

The outer automorphism group is trivial and the Schur multiplier has order 2.

Co₀ has 4 conjugacy classes of involutions; these collapse to 2 in Co₁, but there are 4-elements in Co₀ that correspond to a third class of involutions in Co₁.

An image of a dodecad has a centralizer of type 2¹¹:M₁₂:2, which is contained in a maximal subgroup of type 2¹¹:M₂₄.

An image of an octad or 16-set has a centralizer of the form 2¹⁺⁸.O₈⁺(2), a maximal subgroup.

The smallest faithful permutation representation of Co₁ is on the 98280 pairs {v,–v} of norm 4 vectors.

There is a matrix representation of dimension 24 over the field ${\displaystyle \mathbb {F} _{2}}$ .

The centralizer of an involution of type 2B in the monster group is of the form 2¹⁺²⁴Co₁.

The Dynkin diagram of the even Lorentzian unimodular lattice II_1,25 is isometric to the (affine) Leech lattice Λ, so the group of diagram automorphisms is split extension Λ,Co₀ of affine isometries of the Leech lattice.

Wilson (1983) found the 22 conjugacy classes of maximal subgroups of Co₁, though there were some errors in this list, corrected by Wilson (1988).

• Co₂
• 3.Suz:2 The lift to Aut(Λ) = Co₀ fixes a complex structure or changes it to the complex conjugate structure. Also, top of Suzuki chain.
• 2¹¹:M₂₄ Image of monomial subgroup from Aut(Λ), that subgroup stabilizing the standard frame of 48 vectors of form (±8,0²³) .
• Co₃
• 2¹⁺⁸.O₈⁺(2) centralizer of involution class 2A (image of octad from Aut(Λ))
• Fi₂₁:S₃ ≈ U₆(2):S₃ The lift to Aut(Λ) is the symmetry group of a coplanar hexagon of 6 type 2 points.
• (A₄ × G₂(4)):2 in Suzuki chain.
• 2²⁺¹²:(A₈ × S₃)
• 2⁴⁺¹².(S₃ × 3.S₆)
• 3².U₄(3).D₈
• 3⁶:2.M₁₂ (holomorph of ternary Golay code)
• (A₅ × J₂):2 in Suzuki chain
• 3¹⁺⁴:2.PSp₄(3).2
• (A₆ × U₃(3)).2 in Suzuki chain
• 3³⁺⁴:2.(S₄ × S₄)
• A₉ × S₃ in Suzuki chain
• (A₇ × L₂(7)):2 in Suzuki chain
• (D₁₀ × (A₅ × A₅).2).2
• 5¹⁺²:GL₂(5)
• 5³:(4 × A₅).2
• 7²:(3 × 2.S₄)
• 5²:2A₅

• Conway, John Horton (1968), "A perfect group of order 8,315,553,613,086,720,000 and the sporadic simple groups", Proceedings of the National Academy of Sciences of the United States of America, 61 (2): 398–400, Bibcode:1968PNAS...61..398C, doi:10.1073/pnas.61.2.398, MR 0237634, PMC 225171, PMID 16591697
• Brauer, R.; Sah, Chih-han, eds. (1969), Theory of finite groups: A symposium, W. A. Benjamin, Inc., New York-Amsterdam, MR 0240186
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• Conway, John Horton (1971), "Three lectures on exceptional groups", in Powell, M. B.; Higman, Graham (eds.), Finite simple groups, Proceedings of an Instructional Conference organized by the London Mathematical Society (a NATO Advanced Study Institute), Oxford, September 1969., Boston, MA: Academic Press, pp. 215–247, ISBN 978-0-12-563850-0, MR 0338152 Reprinted in Conway & Sloane (1999, 267-298)
• Conway, John Horton; Sloane, Neil J. A. (1999), Sphere Packings, Lattices and Groups, Grundlehren der Mathematischen Wissenschaften, vol. 290 (3rd ed.), Berlin, New York: Springer-Verlag, doi:10.1007/978-1-4757-2016-7, ISBN 978-0-387-98585-5, MR 0920369
• Thompson, Thomas M. (1983), From error-correcting codes through sphere packings to simple groups, Carus Mathematical Monographs, vol. 21, Mathematical Association of America, ISBN 978-0-88385-023-7, MR 0749038
• Conway, John Horton; Parker, Richard A.; Norton, Simon P.; Curtis, R. T.; Wilson, Robert A. (1985), Atlas of finite groups, Oxford University Press, ISBN 978-0-19-853199-9, MR 0827219
• Griess, Robert L. Jr. (1998), Twelve sporadic groups, Springer Monographs in Mathematics, Berlin, New York: Springer-Verlag, doi:10.1007/978-3-662-03516-0, ISBN 978-3-540-62778-4, MR 1707296
• Wilson, Robert A. (1983), "The maximal subgroups of Conway's group Co₁", Journal of Algebra, 85 (1): 144–165, doi:10.1016/0021-8693(83)90122-9, ISSN 0021-8693, MR 0723071
• Wilson, Robert A. (1988), "On the 3-local subgroups of Conway's group Co₁", Journal of Algebra, 113 (1): 261–262, doi:10.1016/0021-8693(88)90192-5, ISSN 0021-8693, MR 0928064
• Wilson, Robert A. (2009), The finite simple groups., Graduate Texts in Mathematics 251, vol. 251, Berlin, New York: Springer-Verlag, doi:10.1007/978-1-84800-988-2, ISBN 978-1-84800-987-5, Zbl 1203.20012

• MathWorld: Conway Groups
• Atlas of Finite Group Representations: Co₁ version 2
• Atlas of Finite Group Representations: Co₁ version 3