In a conformal field theory the conditions on correlators can be divided into two steps
for a fixed cobordism the correlators need to depend in a certain way on the choice of conformal structure;
the correlators need to glue correctly underly composition of cobordisms.
The functionals that satisfy the first of these conditions are called conformal blocks . The second condition is called the sewing constraint on conformal blocks.
So conformal blocks are something like “precorrelators” or “potential correlators” of a CFT.
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nlab: FFRS-formalism
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Conformal blocks for self-dual higher gauge theory are discussed in