# Motivation behind the construction of Deligne and Lusztig

If $G$ is a connected reductive group over a finite field $\mathbb{F}_q$ and $T$ is a maximal torus in $G$, the famous construction of Deligne and Lusztig (Annals of Math, 1976) associates representations of $G(\mathbb{F}_q)$ to $1$-dimensional representations of $T(\mathbb{F}_q)$. These representations come from the cohomology of the Deligne-Lusztig variety associated to $G$ and $T$, which admits commuting actions of the groups $G(\mathbb{F}_q)$ and $T(\mathbb{F}_q)$. According to various remarks in the Deligne-Lusztig article, two of their sources of motivation were as follows:

1) The conjecture of Macdonald that a construction of this sort should exist.

2) The example of the Drinfeld curve: if $G=SL_2$ and $T$ is the unique (up to conjugacy) non-split maximal torus in $G$, then $T(\mathbb{F}_q)$ can be identified with the kernel of the norm homomorphism

$$\mathbb{F}_{q^2}^\times\to\mathbb{F}_q^\times$$

and $G(\mathbb{F}_q)$ and $T(\mathbb{F}_q)$ both act naturally on the curve $X$ given by the equation $x^qy-xy^q=1$ in the affine plane over $\overline{\mathbb{F}}_q$. (The group $T(\mathbb{F}_q)$ acts by dilations.) The (first) $\ell$-adic cohomology of $X$ realizes all cuspidal irreducible representations of $SL_2(\mathbb{F}_q)$.

In their 1976 paper Deligne and Lusztig give two different constructions of (what later became known as) Deligne-Lusztig varieties (and proved that they are equivalent). The Drinfeld curve turns out to be a special case. However, to me it seems like the jump from the example of the Drinfeld curve (and MacDonald's conjecture) to either of the two general constructions of Deligne-Lusztig is absolutely giant. I was wondering if someone has some additional insight into how the construction was invented.

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If I understand correctly, Deligne-Lusztig theory can be thought of as a generalization of Harish-Chandra induction. The original HCI only applies to rational parabolic subgroups and DLI extends it to non-rational parabolics. So I guess the known properties of HCI would have helped Deligne and Lusztig create their theory.... I still think that the leap remains giant and that the ability to make this leap signifies genius - what more can be said? –  Nick Gill Oct 12 '12 at 15:15
This is true, although the D-L varieties corresponding to usual H-C induction are finite sets, which makes the leap even bigger, in some sense! I am in no way trying to downplay the genius of Deligne and Lusztig; however, there are many other very remarkable constructions in mathematics that do have intuitive explanations behind them. For a related example, look at Lusztig's theory of character sheaves: their definition is also not at all obvious, but it was motivated by a huge amount of concrete calculations, unlike the Deligne-Lusztig theory, which appears to have grown out of one example. –  senti_today Oct 12 '12 at 17:22
I'll attempt an "answer" but should also point out that the spelling is "Macdonald". (A group theorist named Ian D. MacDonald also appears in the literature but is not the more famous Ian G. Macdonald.) –  Jim Humphreys Oct 14 '12 at 20:37