PSU(4,2) - Knowlpedia

$\operatorname{PSU}_{4}(2)$ is the following finite group, with $n=4$ and $q=2$ :

\operatorname{PSU}_{4}(2)=\operatorname{SU}_{4}(2)/\{\lambda I:\lambda^{2+1}=1,\ \lambda^{4}=1\}, \quad \operatorname{SU}_{4}(2)=\{A\in M_{4}(\mathbb F_{2^2}): (A^{(2)})^{\mathsf T}A=I,\ \det A=1\}.

Here $A^{(2)}$ is entrywise $2$ -th power. The matrix field has $2^2$ elements; its involution has fixed field of size $2$ . The scalar $\lambda$ belongs to $\mathbb F_{2^2}^\times$ ; multiplication is multiplication of scalar cosets.

Order and simplicity

Its order is

|\operatorname{PSU}_{4}(2)|=25920.

It is a nonabelian simple group. The exact order is 25,920.

Parameter convention

The parameter $q$ is the size of the fixed field, while matrices have entries in $\mathbb F_{q^2}$ . The Hermitian form is $h(v,w)=\sum_i v_i^q w_i$ . All nondegenerate Hermitian forms of this dimension over the finite field give isomorphic groups. Type ${}^2A_{n-1}$ has absolute rank $n-1$ ; it is not a unitary group over the real or complex numbers.

Small-group identification

The exceptional isomorphism is $\operatorname{PSU}_4(2)\cong\operatorname{PSp}_4(3)$ .

References

Peter J. Cameron, Notes on Classical Groups, §§5.2–5.3, pp. 59–63; PSU(2,q), PSU(3,2), and the PSU(4,2) identification.
Robert A. Wilson, Classical groups (lecture notes), §3.1.7, pp. 5–6; §§3.2–3.4, pp. 7–17.