Is the representation of finite simple groups fully understood? To clarify, I mean have all the simple representations (even finite dimensional) been classified in terms of some classifying set, such as we have for simple Lie algebras/groups.
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$\begingroup$ All irreducible representations of a finite group are finite-dimensional. I am fairly sure they have been classified for alternating and sporadic groups, but I don't know for the ones of Lie type. $\endgroup$– WojowuCommented Apr 25, 2021 at 13:45
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1$\begingroup$ Representation: over what kind of fields? algebraically closed? characteristic zero? $\endgroup$– YCorCommented Apr 25, 2021 at 13:54
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$\begingroup$ I guess over $\mathbb{R}$ and $\mathbb{C}$, but I would be interested to hear about finite fields as well. $\endgroup$– Dick JohnsonCommented Apr 25, 2021 at 14:16
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$\begingroup$ I think the character tables over C for the sporadic groups are in the Atlas and there is a whole book by Carter I believe on the representation theory of the finite groups of Lie Type. I think the alternating groups is almost the same as the symmetric groups via Clifford theory. $\endgroup$– Benjamin SteinbergCommented Apr 25, 2021 at 14:25
1 Answer
A term that may fit in the scope of this problem is "generic character table", character tables of a whole family of groups of Lie type.
Example: Generic character table of $SL_2(q)$, $q = 2^f$
Representations | $I$ | $U$ | $S(a)$ | $T(b)$ |
---|---|---|---|---|
Trivial | $1$ | $1$ | $1$ | $1$ |
Steinberg | $q$ | $0$ | $1$ | $-1$ |
Principal indexed by $k=1 \dots q/2 − 1$ | $q+1$ | $1$ | $\epsilon^{ak}$$+$$\epsilon^{−ak}$ | $0$ |
Discrete indexed by $l=1 \dots q/2$ | $q-1$ | $-1$ | $0$ | $−\eta^{bl}$$−$$\eta^{−bl}$ |
where $ \epsilon = \exp (2πi/(q − 1))$, $η = exp (2πi/(q + 1))$.
The generic character tables of the groups of Lie type $D_5(q)$ and $E_6(q)$ are still unknown, let alone $E_7(q)$ and $E_8(q)$.
And even if we worked out the whole character table, there is still a gap from the character table to representations. To see this, you can try to derive the representations from the character table above.
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3$\begingroup$ What does Deligne-Lusztig theory tell us about $D_5(q)$ and $E_6(q)$? $\endgroup$ Commented Apr 25, 2021 at 19:12