A succession of bigraded pages with . In the cohomological convention, the differential has bidegree . A filtered complex produces such successive approximations to its cohomology.
A spectral sequence differential is the square-zero map acting on one page of a spectral sequence. In the cohomological convention it has bidegree :Its total degree is one. The next page is its cohomology:A class survives to the next page precisely when its differential vanishes, modulo the classes hit by incoming differentials.
For example, if the only nonzero groups on a page are copies of a field at and and the differential between them is an isomorphism, both disappear on the next page. In the homological convention, the differential instead has bidegree and total degree minus one. Specifying the convention prevents reversing the arrows when reading a spectral-sequence diagram.
For a bounded complex of projective right modules and a left module of finite projective dimension, a finite projective resolution of yields a bounded double cochain complex. Its two spectral sequences give the displayed abutment. Over a noncommutative ring, the handedness of the two tensor factors is essential.
If only the columns are nonzero, a cohomological differential of bidegree has no possible nonzero source-target pair for . Projective boundaries in a complex of projectives give this condition because every cohomology module has projective dimension at most one.
The stable page identifies the associated graded module of a filtration of the abutment. For a finite filtration this is an exhaustive, separated finite extension description. It does not itself determine a canonical splitting. Unbounded filtrations require further convergence hypotheses.
A finite filtration of a cochain complex produces a convergent spectral sequence. The induced abutment filtration is the image of . Finiteness in each degree suffices; a bounded range of cochain degrees is not required.
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A spectral sequence is a mathematical tool used in algebraic topology, homological algebra, and related fields to compute homology or cohomology groups that may be difficult to compute directly. It provides a method to systematically approximate these groups through a sequence of pages (typically indexed by integers) and associated differentials.