Bourbaki

Example

Tag 0686, permanent. Cite it as [Bourbaki, Tag 0686].

Algebra, chapter III, § 11. Cogebras, products of multilinear forms, inner products and duality

In particular the above may be applied when E is one of the graded algebras T(M), S(M) or ⋀(M)\bigwedge(M) for an A-module M and P an A-module (with the trivial graduation). To find explicitly the bimodule structures thus obtained, note that the elements of degree −n-n of Homgr_A(T(M), P) (resp. Homgr_A(S(M), P), resp. Homgr_A(⋀(M)\bigwedge(M), P)) are identified with the n-linear mappings (resp. symmetric n-linear mappings, resp. alternating n-linear mappings) of MnM^n into P. It suffices to express the products

f⊔(x1⊗x2⊗⋯⊗xp) f \sqcup (x_1 \otimes x_2 \otimes \cdots \otimes x_p)

(resp. f⊔(x1x2…xp)f \sqcup (x_1 x_2 \ldots x_p)), resp. f⊔(x1∧x2∧⋯∧xp)f \sqcup (x_1 \wedge x_2 \wedge \cdots \wedge x_p)) for every finite sequence (xi)1≤i≤p(x_i)_{1 \leq i \leq p} of elements of M and the analogues for the left inner product. It follows immediately from the definitions that

f⊔(x1⊗x2⊗⋯⊗xp)=(x1⊗x2⊗⋯⊗xp)⊔f=0 f \sqcup (x_1 \otimes x_2 \otimes \cdots \otimes x_p) = (x_1 \otimes x_2 \otimes \cdots \otimes x_p) \sqcup f = 0

if p>np > n and that, for p≤np \leq n, f⊔(x1⊗x2⊗⋯⊗xp)f \sqcup (x_1 \otimes x_2 \otimes \cdots \otimes x_p) (resp.

(x1⊗x2⊗⋯⊗xp)⊔f) (x_1 \otimes x_2 \otimes \cdots \otimes x_p) \sqcup f)

is the (n - p)-linear mapping defined by

{(f⊔(x1⊗x2⊗⋯⊗xp))(y1,…,yn−p)=f(x1,…,xp,y1,…,yn−p)((x1⊗x2⊗⋯⊗xp)⊔f)(y1,…,yn−p)=f(y1,…,yn−p,x1,…,xp). \begin{cases} (f \sqcup (x_1 \otimes x_2 \otimes \cdots \otimes x_p))(y_1, \ldots, y_{n-p}) \\ \hspace{10cm} = f(x_1, \ldots, x_p, y_1, \ldots, y_{n-p}) \\ ((x_1 \otimes x_2 \otimes \cdots \otimes x_p) \sqcup f)(y_1, \ldots, y_{n-p}) \\ \hspace{10cm} = f(y_1, \ldots, y_{n-p}, x_1, \ldots, x_p). \end{cases}

For p>np > n, there are also in Homgr_A(S(M), P) (resp. Homgr_A(⋀(M)\bigwedge(M), P)) formulae (44) with x1⊗x2⊗⋯⊗xpx_1 \otimes x_2 \otimes \cdots \otimes x_p replaced by x1x2…xpx_1 x_2 \ldots x_p (resp. x1∧x2∧⋯∧xpx_1 \wedge x_2 \wedge \cdots \wedge x_p). For p≤np \leq n, the same substitutions in (45) define the symmetric (n - p)-linear mappings f⊔(x1x2…xp)f \sqcup (x_1 x_2 \ldots x_p) and (x1x2…xp)⊔f(x_1 x_2 \ldots x_p) \sqcup f (resp. the alternating (n - p)-linear mappings

f⊔(x1∧x2∧⋯∧xp)and(x1∧x2∧⋯∧xp)⊔f). f \sqcup (x_1 \wedge x_2 \wedge \cdots \wedge x_p) \quad \text{and} \quad (x_1 \wedge x_2 \wedge \cdots \wedge x_p) \sqcup f).

When n=pn = p, the above products are equal to the constant function on MM equal to f(x1,…,xp)f(x_1, \ldots, x_p).

If u:M→Nu : M \to N is an AA-module homomorphism, T(u):T(M)→T(N)T(u) : T(M) \to T(N) is a graded AA-algebra homomorphism, then it follows from what we have seen above that (T(u))∼(T(u))^\sim is a T(M)T(M)-homomorphism of the (T(N),T(N))(T(N), T(N))-bimodule HomgrA(T(N),P)\mathrm{Homgr}_A(T(N), P) into the (T(M),T(M))(T(M), T(M))-bimodule HomgrA(T(M),P)\mathrm{Homgr}_A(T(M), P), relative to the ring homomorphism T(u)T(u). There are analogous results for (S(u))∼(S(u))^\sim and (∧(u))∼(\wedge(u))^\sim.

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

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