Peirce decomposition for associative algebras
If ''e'' is a commuting idempotent (''e''2 = ''e'' and ''e'' is in the center of ''A'') in an associative algebra ''A'', then the two-sided Peirce decomposition writes ''A'' as the direct sum of ''eAe'', ''eA''(1 − ''e''), (1 − ''e'')''Ae'', and (1 − ''e'')''A''(1 − ''e''). There are also left and right Peirce decompositions, where the left decomposition writes ''A'' as the direct sum of ''eA'' and (1 − ''e'')''A'', and the right one writes ''A'' as the direct sum of ''Ae'' and ''A''(1 − ''e''). More generally, if ''e''1, ..., ''e''''n'' are mutually orthogonal idempotents with sum 1, then ''A'' is the direct sum of the spaces ''e''''i''''Ae''''j'' for 1 ≤ ''i'', ''j'' ≤ ''n''.Blocks
An idempotent of a ring is called central if it commutes with all elements of the ring. Two idempotents ''e'', ''f'' are called orthogonal if ''ef'' = ''fe'' = 0. An idempotent is called primitive if it is nonzero and cannot be written as the sum of two orthogonal nonzero idempotents. An idempotent ''e'' is called a block or centrally primitive if it is nonzero and central and cannot be written as the sum of two orthogonal nonzero central idempotents. In this case the ideal ''eR'' is also sometimes called a block. If the identity 1 of a ring ''R'' can be written as the sum :1 = ''e''1 + ... + ''e''''n'' of orthogonal nonzero centrally primitive idempotents, then these idempotents are unique up to order and are called the blocks or the ring ''R''. In this case the ring ''R'' can be written as a direct sum :''R'' = ''e''1''R'' + ... + ''e''''n''''R'' of indecomposable rings, which are sometimes also called the blocks of ''R''.References
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* {{springer, title=Peirce decomposition, id=p/p071970