PΩ(5,q) - Knowlpedia

For $n=2$ and prime powers $q\geq 3$ , $\operatorname{P}\Omega_{5}(q)$ is the projective orthogonal derived group of $Q(x,y,z)=\sum_{i=1}^{2}x_i y_i+z^2$ on $\mathbb F_q^{5}$ :

\Omega=\operatorname O(Q)',\qquad \operatorname{P}\Omega_{5}(q)=\Omega/(\Omega\cap\{\lambda I:\lambda\in\mathbb F_q^\times\}).

Here $\operatorname O(Q)$ is the group of invertible linear maps preserving $Q$ , and its prime denotes the commutator subgroup. Multiplication is multiplication of scalar cosets. The displayed quadratic form, not merely its polar form, is part of the definition.

Order and simplicity

Its order is

|\operatorname{P}\Omega_{5}(q)|=\frac{q^{2^2}\prod_{i=1}^{2}(q^{2i}-1)}{\gcd(2,q-1)}.

It is a nonabelian simple group for every admitted parameter.

Rank and form convention

The ambient quadratic-space dimension is $5$ , and the absolute rank is $2$ . The Witt index is $n$ . For even $q$ , the polar form has a one-dimensional radical and passage to its quotient gives $\operatorname{P}\Omega_{2n+1}(q)\cong\operatorname{PSp}_{2n}(q)$ in this stated range. For odd $q$ and $n\geq3$ , the two groups have equal orders but are not isomorphic.

Low-rank identification

$\operatorname{P}\Omega_5(q)\cong\operatorname{PSp}_4(q)$ for $q\geq3$ . The lower parameters are excluded here because the full orthogonal commutator subgroup can differ from the usual small Chevalley-group convention.

References

Peter J. Cameron, Notes on Classical Groups, §6, pp. 64–74, especially Theorems 6.3 and 6.6; §7 introduction, p. 75.
Robert A. Wilson, Classical groups (lecture notes), §3.1.7, pp. 5–6; §§3.2–3.4, pp. 7–17.