Covering groups of the alternating and symmetric groups

In the mathematical area of group theory, the covering groups of the alternating and symmetric groups are groups that are used to understand the projective representations of the alternating and symmetric groups. The covering groups were classified in (Schur 1911): for n≥4 the covering groups are 2-fold covers except for the alternating groups of degree 6 and 7 where the covers are 6-fold.

For example the binary icosahedral group covers the icosahedral group, an alternating group of degree 5, and the binary tetrahedral group covers the tetrahedral group, an alternating group of degree 4.

Definition and classification

A group homomorphism from D to G is said to be a Schur cover of the finite group G if:

1. the kernel is contained both in the center and the derived subgroup of D, and
2. amongst all such homomorphisms, this D has maximal size.

The Schur multiplier of G is the kernel of any Schur cover and has many interpretations. When the homomorphism is understood, the group D is often called the Schur cover or Darstellungsgruppe.

The Schur covers of the symmetric and alternating groups were classified in (Schur 1911). The symmetric group of degree n ≥ 4 has two isomorphism classes of Schur covers, both of order 2⋅n!, and the alternating group of degree n has one isomorphism class of Schur cover, which has order n! except when n is 6 or 7, in which case the Schur cover has order 3⋅n!.

Finite presentations

Schur covers can be described using finite presentations. The symmetric group Sn has a presentation on n−1 generators ti for i = 1, 2, ..., n−1 and relations

titi = 1, for 1 ≤ in−1
ti+1titi+1 = titi+1ti, for 1 ≤ in−2
tjti = titj, for 1 ≤ i < i+2 ≤ jn−1.

These relations can be used to describe two non-isomorphic covers of the symmetric group. One covering group ${\displaystyle 2\cdot S_{n}^{-}}$ has generators z, t1, ..., tn−1 and relations:

zz = 1
titi = z, for 1 ≤ in−1
ti+1titi+1 = titi+1ti, for 1 ≤ in−2
tjti = titjz, for 1 ≤ i < i+2 ≤ jn−1.

The same group ${\displaystyle 2\cdot S_{n}^{-}}$ can be given the following presentation using the generators z and si given by ti or tiz according as i is odd or even:

zz = 1
sisi = z, for 1 ≤ in−1
si+1sisi+1 = sisi+1siz, for 1 ≤ in−2
sjsi = sisjz, for 1 ≤ i < i+2 ≤ jn−1.

The other covering group ${\displaystyle 2\cdot S_{n}^{+}}$ has generators z, t1, ..., tn−1 and relations:

zz = 1, zti = tiz, for 1 ≤ in−1
titi = 1, for 1 ≤ in−1
ti+1titi+1 = titi+1tiz, for 1 ≤ in−2
tjti = titjz, for 1 ≤ i < i+2 ≤ jn−1.

The same group ${\displaystyle 2\cdot S_{n}^{+}}$ can be given the following presentation using the generators z and si given by ti or tiz according as i is odd or even:

zz = 1, zsi = siz, for 1 ≤ in−1
sisi = 1, for 1 ≤ in−1
si+1sisi+1 = sisi+1si, for 1 ≤ in−2
sjsi = sisjz, for 1 ≤ i < i+2 ≤ jn−1.

Sometimes all of the relations of the symmetric group are expressed as (titj)mij = 1, where mij are non-negative integers, namely mii = 1, mi,i+1 = 3, and mij = 2, for 1 ≤ i < i+2 ≤ jn−1. The presentation of ${\displaystyle 2\cdot S_{n}^{-}}$ becomes particularly simple in this form: (titj)mij = z, and zz = 1. The group ${\displaystyle 2\cdot S_{n}^{+}}$ has the nice property that its generators all have order 2.

Projective representations

Covering groups were introduced by Issai Schur to classify projective representations of groups. A (complex) linear representation of a group G is a group homomorphism G → GL(n,C) from the group G to a general linear group, while a projective representation is a homomorphism G → PGL(n,C) from G to a projective linear group. Projective representations of G correspond naturally to linear representations of the covering group of G.

The projective representations of alternating and symmetric groups are the subject of the book (Hoffman & Humphreys 1992).

Integral homology

Covering groups correspond to the second group homology group, H2(G,Z), also known as the Schur multiplier. The Schur multipliers of the alternating groups An (in the case where n is at least 4) are the cyclic groups of order 2, except in the case where n is either 6 or 7, in which case there is also a triple cover. In these cases, then, the Schur multiplier is the cyclic group of order 6, and the covering group is a 6-fold cover.

H2(An,Z) = 0 for n ≤ 3
H2(An,Z) = Z/2Z for n = 4, 5
H2(An,Z) = Z/6Z for n = 6, 7
H2(An,Z) = Z/2Z for n ≥ 8

For the symmetric group, the Schur multiplier vanishes for n ≤ 3, and is the cyclic group of order 2 for n ≥ 4:

H2(Sn,Z) = 0 for n ≤ 3
H2(Sn,Z) = Z/2Z for n ≥ 4

Construction of double covers

The double cover of the alternating group can be constructed via the spin representation that covers the usual linear representation of the alternating group.

The double covers can be constructed as spin (respectively, pin) covers of faithful, irreducible, linear representations of An and Sn. These spin representations exist for all n, but are the covering groups only for n≥4 (n≠6,7 for An). For n≤3, Sn and An are their own Schur covers.

The alternating group, symmetric group, and their double covers are related in this way, and have orthogonal representations and covering spin/pin representations in the corresponding diagram of orthogonal and spin/pin groups.

Explicitly, Sn acts on the n-dimensional space Rn by permuting coordinates (in matrices, as permutation matrices). This has a 1-dimensional trivial subrepresentation corresponding to vectors with all coordinates equal, and the complementary (n−1)-dimensional subrepresentation (of vectors whose coordinates sum to 0) is irreducible for n≥4. Geometrically, this is the symmetries of the (n−1)-simplex, and algebraically, it yields maps ${\displaystyle A_{n}\hookrightarrow \operatorname {SO} (n-1)}$ and ${\displaystyle S_{n}\hookrightarrow \operatorname {O} (n-1)}$ expressing these as discrete subgroups (point groups). The special orthogonal group has a 2-fold cover by the spin group ${\displaystyle \operatorname {Spin} (n)\to \operatorname {SO} (n),}$ and restricting this cover to ${\displaystyle A_{n}}$ and taking the preimage yields a 2-fold cover ${\displaystyle 2\cdot A_{n}\to A_{n}.}$ A similar construction with a pin group yields the 2-fold cover of the symmetric group: ${\displaystyle \operatorname {Pin} _{\pm }(n)\to \operatorname {O} (n).}$ As there are two pin groups, there are two distinct 2-fold covers of the symmetric group, 2⋅Sn±, also called ${\displaystyle {\tilde {S}}_{n}}$ and ${\displaystyle {\hat {S}}_{n}}$.

Construction of triple cover for n = 6, 7

Main article: Valentiner group

The triple covering of ${\displaystyle A_{6},}$ denoted ${\displaystyle 3\cdot A_{6},}$ and the corresponding triple cover of ${\displaystyle S_{6},}$ denoted ${\displaystyle 3\cdot S_{6},}$ can be constructed as symmetries of a certain set of vector in a complex 6-space. While the exceptional triple covers of A6 and A7 extend to extensions of S6 and S7, these extensions are not central and so do not form Schur covers.

This construction is important in the study of the sporadic groups, and in much of the exceptional behavior of small classical and exceptional groups, including: construction of the Mathieu group M24, the exceptional covers of the projective unitary group ${\displaystyle U_{4}(3)}$ and the projective special linear group ${\displaystyle L_{3}(4),}$ and the exceptional double cover of the group of Lie type ${\displaystyle G_{2}(4).}$

Exceptional isomorphisms

For low dimensions there are exceptional isomorphisms with the map from a special linear group over a finite field to the projective special linear group.

For n = 3, the symmetric group is SL(2,2) ≅ PSL(2,2) and is its own Schur cover.

For n = 4, the Schur cover of the alternating group is given by SL(2,3) → PSL(2,3) ≅ A4, which can also be thought of as the binary tetrahedral group covering the tetrahedral group. Similarly, GL(2,3) → PGL(2,3) ≅ S4 is a Schur cover, but there is a second non-isomorphic Schur cover of S4 contained in GL(2,9) – note that 9=32 so this is extension of scalars of GL(2,3). In terms of the above presentations, GL(2,3) ≅ Ŝ4.

For n = 5, the Schur cover of the alternating group is given by SL(2,5) → PSL(2,5) ≅ A5, which can also be thought of as the binary icosahedral group covering the icosahedral group. Though PGL(2,5) ≅ S5, GL(2,5) → PGL(2,5) is not a Schur cover as the kernel is not contained in the derived subgroup of GL(2,5). The Schur cover of PGL(2,5) is contained in GL(2,25) – as before, 25=52, so this extends the scalars.

For n = 6, the double cover of the alternating group is given by SL(2,9) → PSL(2,9) ≅ A6. While PGL(2,9) is contained in the automorphism group PΓL(2,9) of PSL(2,9) ≅ A6, PGL(2,9) is not isomorphic to S6, and its Schur covers (which are double covers) are not contained in nor a quotient of GL(2,9). Note that in almost all cases, ${\displaystyle S_{n}\cong \operatorname {Aut} (A_{n}),}$ with the unique exception of A6, due to the exceptional outer automorphism of A6. Another subgroup of the automorphism group of A6 is M10, the Mathieu group of degree 10, whose Schur cover is a triple cover. The Schur covers of the symmetric group S6 itself have no faithful representations as a subgroup of GL(d,9) for d≤3. The four Schur covers of the automorphism group PΓL(2,9) of A6 are double covers.

For n = 8, the alternating group A8 is isomorphic to SL(4,2) = PSL(4,2), and so SL(4,2) → PSL(4,2), which is 1-to-1, not 2-to-1, is not a Schur cover.

Properties

Schur covers of finite perfect groups are superperfect, that is both their first and second integral homology vanish. In particular, the double covers of An for n ≥ 4 are superperfect, except for n = 6, 7, and the six-fold covers of An are superperfect for n = 6, 7.

As stem extensions of a simple group, the covering groups of An are quasisimple groups for n ≥ 5.