Errata for Course Notes - J.S. Milne, Top
This file contains miscellaneous errata and additional remarks for my course notes that I haven't yet incorporated into the versions on the web.
Most are taken from e-mail messages -- I thank everyone who has contributed!

Group Theory
Fields and Galois Theory
Algebraic Geometry
Algebraic Number Theory
Modular Functions and Modular Forms
Elliptic Curves
Abelian Varieties
Lectures on Etale Cohomology
Class Field Theory
Complex Multiplication
Algebraic Groups
Basic Theory of Affine Group Schemes
Lie Algebras, Algebraic Groups, and Lie Groups
Reductive Groups
Algebraic groups, Lie groups, and their arithmetic subgroups

## Group Theory v3.13 (GT)

p.40, Exercise 2-5. $Q_n/Z(Q_n)$ =‹$a,b | a^{2^{(n-1)}}, (a^{2^{(n-2)}})b^{-2}, abab^{-1} \textrm{ and } a^{2^{(n-2)}}$›
=‹$a,b | a^{2^{(n-2)}}, b^2, abab$›
$=D_{2^{(n-2)}}$ (not $D_{2^{(n-1)}}$)
or, order on left side is $(2^n)/2=2^{(n-1)}$ while that on right side is $2^n$ so they don't fit. (Yunghyun Ahn)

No known errors.

No known errors.

No known errors.

No known errors.

## Algebraic Groups v1.00 (iAG)

19.56 is misstated: It should say that every split reductive group... T is a split torus.

## Basic Theory of Affine Group Schemes v1.00 (AGS)

p.30. The definition of a sub-coalgebra doesn't make sense unless, e.g., k is a field, because in general D\otimes D is not a submodule of C\otimes C. (Roland Loetscher).

p.261. For an improved exposition of the proof of Theorem 5.1, see RG I, 1.29.

p.259. There is a problem is with my definition of "almost-simple". Certainly "almost-simple" should imply "semisimple", so we should define an almost-simple algebraic group to be a noncommutative smooth connected algebraic group that is semisimple (so its radical over the algebraic closure is trivial) and has no nontrivial proper smooth connected normal algebraic subgroup. The quotient of such a group by its centre should be called simple. (The centre of a reductive group is the kernel of the adjoint representation on the Lie algebra. It suffices to check this over an algebraically closed field, see RG 2.12).

If we don't require that G be semisimple, then we get to the land of pseudo-reductive groups (Conrad-Gabber-Prasad), which is very complicated. Tits defines a pseudo-simple algebraic group to be a noncommutative smooth connected affine algebraic group with no nontrivial proper smooth connected normal subgroup. (Sebastian Petersen)

From Bhupendra Nath Tiwari: list of misprints for AGS, LAG, RG odt; doc

## Lie Algebras, Algebraic Groups, and Lie Groups, v2.00 (LAG)

p.15. In the final paragraph before "The isomorphism theorems", it states that if a is a characteristic ideal of g, then every ideal in a is also an ideal in g. It should read: If a is an ideal in g, then every characteristic ideal in a is also an ideal in g (which is characteristic if a is characteristic).

From David Calderbank
Page 52, Weyl's Theorem 5.20. Part (a) states "If ad is semisimple, then g is semisimple". However, semisimplicity of the adjoint representation is characteristic of reductive Lie algebras (Proposition 6.2), not just semisimple ones. The proof uses more than semisimplicity of the adjoint representation.

Page 56, Proposition 5.29. In the non-algebraically closed case, I don't think it is necessarily true that a Lie algebra with all elements semisimple is commutative. Indeed, so(3) over R would be a counterexample.

Page 57, Proposition 6.4. I don't see how the proof establishes the implication "g has a faithful semisimple representation => g is reductive".

## Reductive Groups, v1.00 (RG)

p.14, following the statement of 1.29: The centre (and radical) of GL_n consists of the scalar matrices, not the diagonal matrices. (Justin Campbell)

From Timo Keller
p. 21: ... with [A,B] = AB - BA,and <- space missing
p. 25, l.-3: ) missing at the end of the equation

## Algebraic groups, Lie groups, and their arithmetic groups v3.00 (ALA)

p. 340 footnote 4. From Brian Conrad: you point out that a root datum is really an ordered 6-tuple, not an ordered 4-tuple. In case it may be of interest, the last bit of information in the 6-tuple (the bijective map a |---> a^{\vee} from roots to coroots) is uniquely determined by everything else, due to the requirements in the axioms of a root datum -- see Lemma 3.2.4 in the book "Pseudo-reductive groups".

I, Theorem 16.21. The centre of a reductive group is of multiplicative type, but need not be reduced (e.g., SL_p). The correct statement is that the radical of G (which is a torus) is the reduced connected group attached to the centre. (There may be other errors of this type, where I incorrectly translated from the characteristic zero case to the general case.) There will be a new version of the notes probably in March 2012.

p. 372 footnote 16. Chapter II?? should be Chapter III.
p. 172, l.1: "and so is split be a finite" should be "and so is split by a finite" (Timo Keller).