## A linear algebra puzzle

August 2, 2010 by Qiaochu Yuan

I won’t get around to substantive blog posts for at least a few more days, so here is a puzzle.

We usually thinks of groups that occur in nature as permutations of a set which preserve some structure on that set. For example, the general linear group preserves the structure of being an -vector space of dimension .

What structure does the special linear group preserve?

(I have an answer, but I’m curious if it can be stated in a more elementary way.)

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on August 3, 2010 at 12:49 am |wokSome kind of volume and orientation.

on August 3, 2010 at 7:44 pm |Qiaochu YuanThat’s the right idea. Can you be more precise? (In particular, can you phrase your answer in terms of natural data one might attach to an object in the category of -vector spaces, e.g. a morphism?)

on August 3, 2010 at 11:08 pm |TheoThe canonical answer is that SL(n) preserves a (signed) volume form. More precisely, GL(n) has a canonical action on n-dimensional space, and hence on the nth exterior power, a line. The subgroup that fixes this line pointwise is SL(n).

This answer is problemmatic in a few ways. When F has positive characteristic and n is sufficiently large, I think that some care should be taken in defining nth exterior powers. Certainly care must be taken in characteristic 2. Moreover, it’s possible for there to be “too many” points, at least for my taste. Consider, for example, when F is a finite field of size p, and n=p-1. Then all non-zero scalars have nth power equal to 1, which is not what you would expect from the case over R or C. I think that these concerns can be avoided, but I think in the smooth category, so I don’t know the details.

on August 4, 2010 at 5:38 am |Edward KmettIs det(A) = 1 what you are getting at?

on August 4, 2010 at 9:26 am |Qiaochu YuanThis is the standard answer, but it is not a structure on an -vector space.

on August 6, 2010 at 5:27 am |Grigory MSL preserves trivialization of top exterior power (i.e. morphism $F\to\det V$).

on November 25, 2010 at 6:12 am |Todd TrimbleI only just saw this thread, and it seems to me Theo got it right, except I don’t see what is problematic. A “volume form” on an n-dimensional space is a nonzero multilinear map ( the ground field) such that $f(v_1, \ldots, v_n) = 0$ whenever $v_i = v_j$ for any distinct . For any we have $f \circ A^n = \det(A)f$, and this can be proven without divisions by integers greater than 1. So “preserves the volume” if and only if . This works over fields of arbitrary characteristic and, with a slight reformulation, even over general commutative rings (where is a free module of rank n).

on November 25, 2010 at 6:41 am |Qiaochu YuanI think Theo meant that the naive definition of “alternating multilinear map” fails in characteristic (the one where you require and so forth). I’m not sure what his second concern means.

on November 25, 2010 at 11:08 am |Todd TrimbleSure, I figured that.

But you never said what your own answer is!

on November 25, 2010 at 11:19 am |Qiaochu YuanAn identification of the top exterior power with . But this definition (as opposed to yours) seems mildly troubling to me in characteristic .

on November 25, 2010 at 12:23 pm |Todd TrimbleThat was my answer too. I believe the right way to define the n-th exterior power (valid for all characteristics) is to take the n-th tensor power and divide by the subspace of tensors spanned by those where for distinct . (Lang’s Algebra, chapter 19.)