block

《Cohomologie Locale Des Faisceaux Coherents (Sga 2): Seminaire De Geometrie Algebrique Du Bois Marie 1962 (Documents Mathematiques)》 · 印刷第 46—47 页 Section 3.6「The Matrix Summands of the Complex Group Algebra」，Proposition 3.6.1 完整陈述及其后对 blocks 的命名句

Proposition 3.6.1. Let $A$ be a ring with identity. Decompositions $A=A_1\oplus\cdots\oplus A_r$ as direct sums of $2$-sided ideals $A_i$ biject with expressions $1=e_1+\cdots+e_r$ as a sum of orthogonal central idempotent elements, in such a way that $e_i$ is the identity element of $A_i$ and $A_i=Ae_i$. The $A_i$ are indecomposable as rings if and only if the $e_i$ are primitive central idempotent elements. If every $A_i$ is indecomposable as a ring then the subsets $A_i$ and also the primitive central idempotents $e_i$ are uniquely determined; furthermore, every central idempotent can be written as a sum of certain of the $e_i$. […] The primitive central idempotents, and also the indecomposable ring summands to which they correspond according to Proposition 3.6.1, are known as blocks.