Bourbaki

Corollary 1

Tag 0612, permanent. Cite it as [Bourbaki, Tag 0612].

Algebra, chapter III, § 4. Tensor products of algebras

In the notation of Proposition 10, suppose further that F is a graded A-algebra of type Δ\Delta and that each fif_i is a graded algebra homomorphism relative to ϕi:Δi→Δ\phi_i : \Delta_i \to \Delta; then f=ε⨂ifif = \varepsilon \bigotimes_i f_i is a graded algebra homomorphism.

This follows immediately from the definition of ff and the fact that

∑i∈Iϕi(αi)=(αi) \sum_{i \in I} \phi_i(\alpha_i) = (\alpha_i)
by definition of the ϕi\phi_i.

It is therefore seen that (E,ψ)(E, \psi) is also a solution of another universal mapping problem, where this time Σ\Sigma is the species of graded A-algebra structure of type Δ\Delta, the morphisms being graded algebra homomorphisms of type Δ\Delta and the α\alpha-mappings the mappings ∏ifi\prod_i f_i, where, in addition to conditions (26), it is assumed that fif_i is a graded algebra homomorphism relative to ϕi\phi_i.

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

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