- 1 Definition for commutative rings
- 2 Extra structure
- 3 Functoriality
- 4 Preservation of structure and properties
- 5 Related notions
Definition for commutative rings
Definition with symbols
Let denote a commutative unital ring. The, the polynomial ring over in one variable, denoted as where is termed the indeterminate, is defined as the ring of formal polynomials in with coefficients in .
Note that we could choose any letter for the indeterminate in place of and our notation would change accordingly. For instance, if we use the letter for the indeterminate, the ring is denoted .
The polynomial ring over any commutative unital ring is, first and foremost, a commutative unital ring. However, it has a number of additional structures, some of which are described below.
As an algebra over the original ring
The polynomial ring is a -algebra. In fact, any -algebra generated by one element over , is a quotient, as a -algebra, of .
As a graded ring
The polynomial ring comes with a natural gradation. The graded component of the polynomial ring is the -span of .
In fact, this makes a connected graded -algebra.
As a filtered ring
The polynomial ring comes with a natural filtration. The filtered component of the polynomial ring is the subgroup comprising the polynomials of degree at most . This is the filtration corresponding to the gradation described above, and makes a connected filtered -algebra.
The polynomial ring can be viewed as a functor in any of the following senses:
- A functor from the category of commutative unital rings to itself
- A functor from the category of commutative unital rings to the category of graded rings
- A functor from the category of commutative unital rings to the category of filtered rings
Preservation of structure and properties
For many properties of commutative unital rings, it is true that when we pass to the polynomial ring, the property is preserved. However, this is not true for all properties of interest. For a list of properties that are preserved on passing to the polynomial ring, refer:
Variations that are the same over a field of characteristic zero
- Ring generated by binomial polynomials (makes sense for any commutative unital ring of characteristic zero)
- Divided polynomial ring
- Ring of integer-valued polynomials (makes sense over any integral domain)
Some important notions include: