Bourbaki

Remark

Tag 063C, permanent. Cite it as [Bourbaki, Tag 063C].

Algebra, chapter III, § 7. Exterior algebras

Let N=⨁λ∈LNλN = \bigoplus_{\lambda \in L} N_\lambda be the direct sum of another family of AA-modules with LL as indexing set and, for all λ∈L\lambda \in L, let vλ:Mλ→Nλv_\lambda : M_\lambda \to N_\lambda be an AA-linear mapping, whence there is an AA-linear mapping v=⨁λvλ:M→Nv = \bigoplus_\lambda v_\lambda : M \to N (II, § 1, no. 6, Proposition 7). Then the diagram

⨂λ∈LΛ(Mλ)→gMΛ(M)↓↓⨂λ∈LΛ(vλ)Λ(v)↓↓⨂λ∈LΛ(Nλ)→gNΛ(N) \begin{array}{ccc} \bigotimes_{\lambda \in L} \Lambda(M_\lambda) & \xrightarrow{g_M} & \Lambda(M) \\ \downarrow & & \downarrow \\ \bigotimes_{\lambda \in L} \Lambda(v_\lambda) & & \Lambda(v) \\ \downarrow & & \downarrow \\ \bigotimes_{\lambda \in L} \Lambda(N_\lambda) & \xrightarrow{g_N} & \Lambda(N) \end{array}

is commutative (cf. § 4, no. 5, Corollary to Proposition 8).

The sub-AA-module of ⨂λ∈LΛ(Mλ)\bigotimes_{\lambda \in L} \Lambda(M_\lambda) with which Λn(M)\Lambda^n(M) is identified by means of the isomorphism gg can be described more precisely. For every finite subset JJ of LL, we write EJ=⨂λ∈JΛ(Mλ)E_J = \bigotimes_{\lambda \in J} \Lambda(M_\lambda), so that ⨂λ∈LΛ(Mλ)=lim⁡→EJ\bigotimes_{\lambda \in L} \Lambda(M_\lambda) = \lim \to E_J relative to the directed set F(L)\mathscr{F}(L) of finite subsets of LL, by definition (§4§ 4, no. 8, Remark 1). For every family ν=(nλ)∈N(L)\nu = (n_\lambda) \in \mathbf{N}^{(L)} (therefore with finite support) such that ∑λ∈Lnλ=n\sum_{\lambda \in L} n_\lambda = n and every finite subset JJ of LL containing the support of the family ν\nu, we write

ΛJ,ν(M)=⨂λ∈JΛnλ(Mλ) \Lambda^{J,\nu}(M) = \bigotimes_{\lambda \in J} \Lambda^{n_\lambda}(M_\lambda)

so that the submodule EJ,nE_{J,n} of elements of degree nn in EJE_J is the direct sum of the ΛJ,ν(M)\Lambda^{J,\nu}(M) for all families ν\nu of support contained in JJ and such that

∑λ∈Lnλ=n \sum_{\lambda \in L} n_\lambda = n

(§4§ 4, no. 7, Proposition 10 and no. 8). By way of convention we write ΛJ,ν(M)={0}\Lambda^{J,\nu}(M) = \{0\} for the families ν\nu whose support is not contained in JJ; then

E_{J,n} can also be called the direct sum of all the ⋀J,ν(M)\bigwedge^{J,\nu}(M), where ν\nu runs through the set HnH_n of all families ν=(nλ)λ∈L\nu = (n_\lambda)_{\lambda \in L} such that ∑λ∈Lnλ=n\sum_{\lambda \in L} n_\lambda = n. Since ⋀0(Mλ)\bigwedge^0(M_\lambda) is identified with AA, clearly also for two finite subsets J⊂J′J \subset J' of LL and a family ν\nu of support contained in JJ, the canonical mapping ⋀J,ν(M)→⋀J′,ν(M)\bigwedge^{J,\nu}(M) \to \bigwedge^{J',\nu}(M) (restriction of the canonical mapping EJ→EJ′E_J \to E_{J'} to ⋀J,ν(M)\bigwedge^{J,\nu}(M)) is bijective. If we write, for all ν∈Hn\nu \in H_n,

⋀ν(M)=lim⁡⟶⋀J,ν(M) \bigwedge^\nu(M) = \lim_{\longrightarrow} \bigwedge^{J,\nu}(M)

it is then seen that, taking account of II, § 6, no. 2, Proposition 5, we have:

Bourbaki puts the proof directly after the statement, so what is above is the statement and its proof together.

Cited by

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Cites

Transcribed from the printed edition, 1998, Springer. Not translated.