4 added 95 characters in body

To keep things simple, let us assume we work over a perfect field. The easiest part of motivic cohomology which we can get is the Picard group (i.e. the Chow group in degree 1). This works essentially like in topology: in the (model) category of simplicial Nisnevich sheaves (over smooth k-schemes), the classifying space of the multiplicative group $\mathbb G_m=\mathbb A^1-\{0\}$ has the $\mathbb A^1$-homotopy type of the infinite dimensional projective space. Moreover, as the Picard group is homotopy invariant for regular schemes (semi-normal is even enough), the fact that $H^1(X,\mathbb G_m)=\text{Pic}(X)$ reads as $[X,B\mathbb G_m]=\text{Pic}(X)=CH^1(X)$, where $[?,?]$ stands for the $\text{Hom}$ in the $\mathbb A^1$-homotopy category of $k$-schemes, denoted by $H(k)$.

In general, we denote by $K(\mathbb Z(n),2n)$ the $n$-th motivic Eilenberg-MacLane space, i.e. the object of $H(k)$ which represents the $n$-th Chow group in $H(k)$: for any smooth $k$-scheme $X$, one has

$$[X,\Omega^i K(\mathbb Z(n),2n)]=H^{2n-i}(X,\mathbb Z(n))$$

(where $\Omega^i$ stands for the $i$-th loop space functor). For $i=0$, we just get the usual Chow groups:

$$H^{2n}(X, \mathbb Z (n)))\simeq CH^n(X) .$$

Then, there are several models for $K(\mathbb Z(n),2n)$, one of the smallest being constructed as follows. What I explained above is that $K(\mathbb Z(1),2)$ is the infinite projective space. $K(\mathbb Z(0),0)$ is simply the constant sheaf $\mathbb Z$. For higher $n$, here is a construction (this is Voevodsky's).

Given a $k$-scheme $X$, denote by $L(X)$ the presheaf with transfers associated to X, that is the presheaf of abellian groups whose sections over a smooth $k$-scheme $V$ are the finite correspondences from $V$ to $X$ (i.e. the finite linear combinations of cycles $\sum n_iZ_i$ in $V \times X$ such that $Z_i$ is finite and surjective over $V$). This is a presheaf, where the pullbacks are defined using the pullbacks of cycles (the condition that the $Z_i$; are finite and surjective over a smooth (hence normal) scheme $V$ makes that this is well defined without working up to rational equivalences, and as we consider only pullbacks along maps $U \to V$ with $U$ and $V$ smooth (hence regular) ensures that the multiplicities which will appear from these pullbacks will always be integers). The presheaf $L(X)$ is a sheaf for the Nisnevich topology. This construction is functorial in $X$ (I will need this functoriality only for closed immersions).

Let $X$ (resp. $Y$) be the cartesian product of $n$ (resp. $n-1$) copies of the projective line. The point at infinity gives a family of $n$ maps $u_i : Y \to X$. Then a model of $K(\mathbb Z(n),2n)$ in $H(k)$ is the sheaf of sets obtained as the quotient (in the category of Nisnech sheaves of abelian groups) of $L(X)$ by the subsheaf generated by the images of the maps $L(u_i):L(Y)\to L(X)$.

If you want a more conceptual definition, there is also the direction of algebraic cobordism (but for this, you need to understand the stable homotopy of $\mathbb P^1$-spectra, but maybe it is enough at first to think of $\mathbb P^1$-spectra simply as the cohomology theories allowed in homotopy theory of schemes): the idea is that there is an algebraic cobordism, which is represented by a $\mathbb P^1$-spectrum $MGL$ (the analog of of the spectrum $MU$ which represents complex cobordism in algebraic topology). The idea is that $MGL$ is the universal oriented cohomology theory (for short, this means that, if a cohomology theory $E$ satisfies the projective bundle formula, then the choice of an orientation, i.e. of a generating class in the second cohomology group of $\mathbb P^1$ with coefficients in $E(1)$, is the same as a map of ring spectra $MGL \to E$. The idea is that, as in algebraic topology, formal groups laws classify oriented cohomology theories ($MGL$ corresponding to the initial formal group law). The cohomology theory which corresponds to the multiplicative formal group law is $KGL$, the $\mathbb P^1$-spectrum which represents algebraic K-theory, while the cohomology theory corresponding to the additive formal group law is motivic cohomology (this latter characterization has been announced by F. Morel and M. Hopkins if $k$ is of characteristic zero, but is not published yet, and it is known for any field $k$ if we work with rational coefficients (this is a result of Spitzweck, Nauman, Ostvaer)).

3 fixed slight inaccuracy

To keep things simple, let us assume we work over a perfect field. The easiest part of motivic cohomology which we can get is the Picard group (i.e. the Chow group in degree 2)1). This works essentially like in topology: in the (model) category of simplicial Nisnevich sheaves (over smooth k-schemes), the classifying space of the multiplicative group $\mathbb G_m=\mathbb A^1-\{0\}$ has the $\mathbb A^1$-homotopy type of the infinite dimensional projective space. Moreover, as the Picard group is homotopy invariant for regular schemes (semi-normal is even enough), the fact that $H^1(X,\mathbb G_m)=\text{Pic}(X)$ reads as $[X,B\mathbb G_m]=\text{Pic}(X)=CH^2(X)$G_m]=\text{Pic}(X)=CH^1(X)$, where$[?,?]$stands for the$\text{Hom}$in the$\mathbb A^1$-homotopy category of$k$-schemes, denoted by$H(k)$. In general, we denote by$K(\mathbb Z(n),2n)$the$n$-th motivic Eilenberg-MacLane space, i.e. the object of$H(k)$which represents the$n$-th Chow group in$H(k)$: for any smooth$k$-scheme$X$, one has $$[s^i(X),K(\mathbb [X,\Omega^i K(\mathbb Z(n),2n)]=H^{2n-i}(X,\mathbb Z(n)) \quad \text{(where Z(n))$$ (where$s^i$\Omega^i$ stands for the $i$-th suspension (in the usual sense))}.$$loop space functor). Then, there are several models for K(\mathbb Z(n),2n), one of the smallest being constructed as follows. What I explained above is that K(\mathbb Z(1),2) is the infinite projective space. K(\mathbb Z(0),0) is simply the constant sheaf \mathbb Z. For higher n, here is a construction (this is Voevodsky's). Given a k-scheme X, denote by L(X) the presheaf with transfers associated to X, that is the presheaf of abellian groups whose sections over a smooth k-scheme V are the finite correspondences from V to X (i.e. the finite linear combinations of cycles \sum n_iZ_i in V \times X such that Z_i is finite and surjective over V). This is a presheaf, where the pullbacks are defined using the pullbacks of cycles (the condition that the Z_i; are finite and surjective over a smooth (hence normal) scheme V makes that this is well defined without working up to rational equivalences, and as we consider only pullbacks along maps U \to V with U and V smooth (hence regular) ensures that the multiplicities which will appear from these pullbacks will always be integers). The presheaf L(X) is a sheaf for the Nisnevich topology. This construction is functorial in X (I will need this functoriality only for closed immersions). Let X (resp. Y) be the cartesian product of n (resp. n-1) copies of the projective line. The point at infinity gives a family of n maps u_i : Y \to X. Then a model of K(\mathbb Z(n),2n) in H(k) is the sheaf of sets obtained as the quotient (in the category of Nisnech sheaves of abelian groups) of L(X) by the subsheaf generated by the images of the maps L(u_i):L(Y)\to L(X). If you want a more conceptual definition, there is also the direction of algebraic cobordism (but for this, you need to understand the stable homotopy of \mathbb P^1-spectra, but maybe it is enough at first to think of \mathbb P^1-spectra simply as the cohomology theories allowed in homotopy theory of schemes): the idea is that there is an algebraic cobordism, which is represented by a \mathbb P^1-spectrum MGL (the analog of of the spectrum MU which represents complex cobordism in algebraic topology). The idea is that MGL is the universal oriented cohomology theory (for short, this means that, if a cohomology theory E satisfies the projective bundle formula, then the choice of an orientation, i.e. of a generating class in the second cohomology group of \mathbb P^1 with coefficients in E(1), is the same as a map of ring spectra MGL \to E. The idea is that, as in algebraic topology, formal groups laws classify oriented cohomology theories (MGL corresponding to the initial formal group law). The cohomology theory which corresponds to the multiplicative formal group law is KGL, the \mathbb P^1-spectrum which represents algebraic K-theory, while the cohomology theory corresponding to the additive formal group law is motivic cohomology (this latter characterization has been announced by F. Morel and M. Hopkins if k is of characteristic zero, but is not published yet, and it is known for any field k if we work with rational coefficients (this is a result of Spitzweck, Nauman, Ostvaer)). 2 latex The easiest part of motivic cohomology which we can get is the Picard group (i.e. the Chow group in degree 2). This works essentially like in topology: in the (model) category of simplicial Nisnevich sheaves (over smooth k-schemes), the classifying space of the multiplicative group G_m=A^1-{0} \mathbb G_m=\mathbb A^1-\{0\} has the A^1-homotopy \mathbb A^1-homotopy type of the infinite dimensionnal dimensional projective space. Moreover, as the picard Picard group is homotopy invariant for regular schemes (semi-normal is even enough), the fact that H^1(X,G_m)=Pic(X) H^1(X,\mathbb G_m)=\text{Pic}(X) reads as [X,BG_m]=Pic(X)=CH^2(X), [X,B\mathbb G_m]=\text{Pic}(X)=CH^2(X), where [?,?] [?,?] stands for the Hom \text{Hom} in the A^1-homotopy \mathbb A^1-homotopy category of k-schemes, k-schemes, denoted by H(k).H(k). In general, we denote by K(Z(n),2n) K(\mathbb Z(n),2n) the nth n-th motivic Eilenberg-MacLane space, i.e. the object of H(k) H(k) which represents the nth n-th Chow group in H(k): H(k): for any smooth k-scheme X, k-scheme X, one has [s^i(X),K(Z(n),2n)]=H^2n-i(X,Z(n)) (where s^i$$[s^i(X),K(\mathbb Z(n),2n)]=H^{2n-i}(X,\mathbb Z(n)) \quad \text{(where $s^i$ stands for the ith $i$-th suspension (in the usual sense)).sense))}.

Then, there are several models for K(Z(n),2n)$K(\mathbb Z(n),2n)$, one of the smallest being constructed as follows. What I explained above is that K(Z(1),2) $K(\mathbb Z(1),2)$ is the infinite projective space.K(Z(0),0) $K(\mathbb Z(0),0)$ is simply the constant sheaf Z$\mathbb Z$. For higher n, $n$, here is a construction (this is Voevodsky's).

Given a k-scheme X, $k$-scheme $X$, denote by L(X) $L(X)$ the presheaf with transfers associated to X, that is the presheaf of abellian groups whose sections over a smooth k-scheme V $k$-scheme $V$ are the finite correspondances correspondences from V $V$ to X $X$ (i.e. the finite linear combinations of cyclesΣᵢ n Z $\sum n_iZ_i$ in $V x X \times X$ such that Z $Z_i$ is finite and surjective over V). $V$). This is a presheaf, where the pullbacks are defined using the pullbacks of cycles (the condition that the Z $Z_i$; are finite and surjective over a smooth (hence normal) scheme V $V$ makes that this is well defined without working up to rational equivalences, and as we consider only pullbacks along maps $U -> V \to V$ with U $U$ and V $V$ smooth (hence regular) ensures that the multiplicities which will appear from these pullbacks will always be integers). The presheaf L(X) $L(X)$ is a sheaf for the Nisnevich topology. This construction is functorial in X $X$ (I will need this functoriality only for closed immersions).

Let X $X$ (resp. Y) $Y$) be the cartesian product of n $n$ (resp. n-1) $n-1$) copies of the projective line.The point at infinity gives a family of n $n$ maps u $u_i : Y -> X\to X$. Then a model of K(Z(n),2n) $K(\mathbb Z(n),2n)$ in H(k) $H(k)$ is the sheaf of sets obtained as the quotient (in the category of Nisnech sheaves of abelian groups) of L(X) $L(X)$ by the subsheaf generated by the images of the maps L(uᵢ):L(Y)->L(X).$L(u_i):L(Y)\to L(X)$.

If you want a more conceptual definition, there is also the direction of algebraic cobordism (but for this, you need to understand the stable homotopy of P^1-spectra$\mathbb P^1$-spectra, but maybe it is enough at first to think of P^1-spectra $\mathbb P^1$-spectra simply as the cohomology theoriesallowed in homotopy theory of schemes): the idea is that there is an algebraic cobordism, which is represented by a P^1-spectrum MGL $\mathbb P^1$-spectrum $MGL$ (the analog of of the spectrum MU $MU$ which represents complex cobordism in algebraic topology). The idea is that MGL $MGL$ is the universal oriented cohomology theory (for short, this means that, if a cohomology theory E $E$ satisfies the projective bundle formula, then the choice of an orientation, i.e. of a generating class in the second cohomology group of P^1 $\mathbb P^1$ with coefficients in E(1), $E(1)$, is the same as a map of ring spectra MGL->E$MGL \to E$. The idea is that, as in algebraic topology, formal groups laws classify oriented cohomology theories (MGL $MGL$ corresponding to the initial formal group law). The cohomology theory which corresponds to the multiplicative formal group law is KGL, $KGL$, the P^1-spectrum $\mathbb P^1$-spectrum which represents algebraic K-theory, while the cohomology theory corresponding to the additive formal group law is motivic cohomology (this latter characterization has been announced by F. Morel and M. Hopkins if k $k$ is of characteristic zero, but is not published yet, and it is known for any field k $k$ if we work with rational coefficients (this is a result of Spitzweck, Nauman, Ostvaer)).

1