# Uniqueness/motivation for the Suslin-Voevodsky theory of relative cycles.

In the Suslin-Voevodsky paper "Relative cycles and Chow sheaves" they define abelian groups $z(X/S, r)$ for schemes $X$ of finite type over Noetherian schemes $S$, and then they show that these groups satisfy a number of properties, for example 1) they are cdh-sheaves in $S$, and 2) the obvious maps $cycl: \mathbb{Z} Hilb(X / S, r) \to z(X/S, r)$ form a natural transformation of presheaves, etc

The only motivation I can think of for the definition of $z(X/S, r)$ is that it gives a "good theory". However, we could also consider, for example, the presheaf obtained as the image of cycl and we would also get a theory of relative cycles with well defined pullback although we would lose some other proprieties. Then there are also the presheaves $z_{equi}(X/S, r), c(X/S, r), c_{equi}(X/S, r)$.

So my question is: (1) is there a list of properties that determines the groups $z(X / S, r)$ uniquely, and (2) why do we want each of these properties.

For (1) the answer is of course yes and so the trick is to find a list of properties that we can justify wanting, apart from saying "they are nice".

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In relation to Theorem 4.2.11, the conditions I had in mind when I mentioned that the answer to (1) was yes, were (i) relative cycles should include the image of cycl, (ii) should satisfy cdh descent, (iii) pullback via blowups is defined via the strict transform, (iv) if the cycle is flat then pullback via k-points is defined in the naive way, (v) arbitrary pullbacks preserve pullbacks by k-points. I believe this to be a list of conditions that completely determines the z(X/S, r), but I don't find it aesthetically pleasing, or very motivational. :-( –  anon Aug 10 '10 at 15:12
Actually, the list can be reduced to (n) z(X/S) is a subgroup of Cycl(X), (i) cycl is a morphism of presheaves, (ii) pullback along a blow-up is calculated using the strict transform. z(X/S) is the largest presheaf satisfying these three properties. This is a nicer list, but I don't really see why these properties, apart from being nice, should have such priority –  anon Aug 10 '10 at 18:14

I will just sum up the situation as I see it (too big for the comment box).

One important goal is to set up a good intersection theory for cycles without quotienting by rational equivalence, and using it to get a composition product for finite correspondences, which are by definition elements of groups of the form $c_{equi}(X\times_S Y/X,0)$

It is true that the variety of definitions of cycle groups in the paper is somewhat confusing. There are 16 possible groups because starting from the "bare" notion of relative cycles (def. 3.1.3) there are 4 binary conditions : being effective, being equidimensional, having compact support (c, PropCycl), and being "special", i.e satisfying the equivalent conditions of lemma 3.3.9 (everything except Cycl and PropCycl). So you have

1)$z_{equi}(X/S,r)\subset z(X/S,r)\subset Cycl(X/S,r) \supset Cycl_{equi}(X/S,r)$

and their effective counter-parts.

2)$c_{equi}(X/S,r)\subset c(X/S,r)\subset PropCycl(X/S,r) \supset PropCycl_{equi}(X/S,r)$

and their effective counter-parts.

(1) is then a "subline" of 2))

In a sense, the most satisfying definition would be to use only cycles which are flat over $S$ (the $\mathbb{Z}Hilb$-groups, or the closely related $z_{equi}$) but pullbacks along arbitrary morphisms are not defined there in general.

With the groups Cycl, thanks to the relative cycle condition built in Cycl, you have pullbacks along arbitrary morphism, but only with rational coefficients (thm 3.3.1, the denominators of the multiplicities are divisible by residue characteristics)

The main interest of the "special" relative cycles $z(-,-)$ is in their definition : they admit integral pullbacks ! Then you have the small miracle that this condition is stable by those pullbacks and you get a subpresheaf. This means that using them you can set up intersection theory with integral coefficients even on singular car p schemes.

All this zoology simplifies when $S$ is nice : there are some results when $S$ is geometrically unibranch, but the nicest case is $S$ regular, in which the chains of inclusions I wrote down collapse, you are left with two distinctions which are reasonable from the point of view of classical intersection theory : effective/non-effective, general/with compact support. Furthermore, the intersection multiplicities are computed by the Tor multiplicity formula, so the Suslin-Voevodsky theory is really an extension of local intersection theory of regular rings as in Serre's book.

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Thanks for the over-abundance of interest. A minor corrections: $\ZZ Hilb(X / S, r)$ is indeed a presheaf and cycl defines a morphism of presheaves into $z_{equi}(X / S, r)$ Corollary 3.3.11, see also Theorem 4.2.11 for another intuitive approach to $z, z^{eff}, c, c^{eff}$. For me, the image of cycl would be the most sensible definition and indeed, many proofs in that paper involve essentially reducing to the case where your cycles are in this image. –  anon Aug 10 '10 at 15:11
You might enjoy reading the following : arxiv.org/abs/0912.2110, section 7 and 8, where all this is recast and used to define $DM(S)$ for general bases $S$ (the 6 operation formalism works for rational coefficients and geometrically unibrach schemes). –  Simon Pepin Lehalleur Aug 10 '10 at 16:06
Its funny you should mention that article. Actually, I co-organised a groupe de travail on the first half of it last year. ;-) –  anon Aug 10 '10 at 16:46
Hmmm, I also co-organized a "groupe de travail" (note the suspiciouly similar french expression) on the first half of it last year... I think we have both met our döppelganger ! And where was that, if you don't mind ? –  Simon Pepin Lehalleur Aug 10 '10 at 17:54