Reduced product - Alchetron, The Free Social Encyclopedia

In model theory, a branch of mathematical logic, and in algebra, the reduced product is a construction that generalizes both direct product and ultraproduct.

Let {S_i | i ∈ I} be a family of structures of the same signature σ indexed by a set I, and let U be a filter on I. The domain of the reduced product is the quotient of the Cartesian product

∏ i ∈ I S i

by a certain equivalence relation ~: two elements (a_i) and (b_i) of the Cartesian product are equivalent if

{ i ∈ I : a i = b i } ∈ U

If U only contains I as an element, the equivalence relation is trivial, and the reduced product is just the original Cartesian product. If U is an ultrafilter, the reduced product is an ultraproduct.

Operations from σ are interpreted on the reduced product by applying the operation pointwise. Relations are interpreted by

R ( ( a i 1 ) / ∼ , … , ( a i n ) / ∼ ) ⟺ { i ∈ I ∣ R S i ( a i 1 , … , a i n ) } ∈ U .

For example, if each structure is a vector space, then the reduced product is a vector space with addition defined as (a + b)_i = a_i + b_i and multiplication by a scalar c as (ca)_i = c a_i.

References

Reduced product Wikipedia

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