polynomials in algebraic systems
Polynomials in an Algebraic System
Let be an algebraic system. Given any non-negative integer , let be a set of functions from to , satisfying the following criteria:
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the projection is an element of for each between and .
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for every -ary operator symbol , the corresponding element is an element of .
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Take the intersection of all such sets and call it . Then is the smallest set satisfying the above two conditions. We call an element of an -ary polynomial of the algebra . A polynomial of is some -ary polynomial of . Two polynomials are said to be equivalent if they have the same arity , and for any , .
Examples.
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Let be a group. Then is a unary polynomial. In general, a unary polynomial of has the form , . For a binary polynomial, first note that and are both binary. For this, a binary polynomial of looks like
where and the rest are positive integers. If is abelian, a binary polynomial has the simplified form , and more generally, an -ary polynomial of an abelian group has the form
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Let be a ring. is a unary polynomial, so are and where are non-negative integers. In general, a unary polynomial of looks like
where are non-negative integers with , and is any positive integer. This is the form of a polynomial that is familiar to most people.
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Continuing from the example above, note that, however,
where is in general not a polynomial under this definition. This is an example of an algebraic function in an algebraic system.
Remarks.
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is non-empty iff contains constant symbols (nullary function symbols).
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By virtue of the definition, is an algebraic system for each non-negative integer .
Polynomial Symbols
If we have two algebras of the same type, then, by the definition above, a polynomial of may be considered as a polynomial of , and vice versa. Putting this formally, we introduce polynomial symbols for a class of algebras of the same type . Let be a countably infinite set of variables, or symbols. For any non-negative integer , consider a set consisting of the following elements:
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,
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every nullary operator symbol,
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if is -ary and , then .
Take the intersection of all such sets and we end up with a set satisfying the two conditions again, call it . Any element of is called an -ary polynomial symbol of . A polynomial symbol of is just some -ary polynomial symbol. In model theory, a polynomial symbol is nothing more than a term (http://planetmath.org/Term) over in the -language.
Now, suppose . For a given non-negative integer , consider the function , defined recursively by
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,
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,
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3.
.
Then by the constructions of and , is a surjection. Any polynomial of is said to be induced by a polynomial symbol of if . We usually write to denote that the polynomial (of ) is induced by the polynomial symbol (of ). It is not hard to see that, between two algebras of the same type, there is a one-to-one correspondence between their respective polynomials the same arities. However, equivalence of polynomials in one algebra does not translate to equivalence in another.
Example. and are both polynomials symbols in the class of lattices. In the subclass of distributive lattices, the induced polynomials are equivalent. However, in the subclass of modular lattices, the equivalence no longer holds.
Remark. Like , is also an algebraic system, called the -ary polynomial algebra, or absolutely free algebra, of . Furthermore, the function above is an algebra homomorphism.
References
- 1 G. Grätzer: Universal Algebra, 2nd Edition, Springer, New York (1978).
- 2 S. Burris, H.P. Sankappanavar: A Course in Universal Algebra, Springer, New York (1981).
Title | polynomials in algebraic systems |
Canonical name | PolynomialsInAlgebraicSystems |
Date of creation | 2013-03-22 16:47:31 |
Last modified on | 2013-03-22 16:47:31 |
Owner | CWoo (3771) |
Last modified by | CWoo (3771) |
Numerical id | 16 |
Author | CWoo (3771) |
Entry type | Definition |
Classification | msc 08A40 |
Synonym | absolutely free algebra |
Related topic | TermAlgebra |
Related topic | AlgebraicFunction |
Defines | polynomial |
Defines | equivalent polynomials |
Defines | polynomial symbol |
Defines | induced polynomial |
Defines | polynomial algebra |