# Need help and confirmation about some notational symbols

• Apr 26th 2012, 09:26 AM
mrproper
Need help and confirmation about some notational symbols
What does $Z[X]/(x^n-1)$ mean?

Z[X] must be the ring of all polynomials with coefficients in Z.
This makes $(x^n-1)$ an ideal ???
And $Z[X]/(x^n-1)$ then is quotient ring?

Is it possible this notation to denote something other ?
• Apr 27th 2012, 01:39 AM
ILikeSerena
Re: Need help and confirmation about some notational symbols
An ideal is a set.
The denominator of a quotient ring also has to be a set.

$(x^n-1)$ is not a set, but an element of Z[X].

So I'd interpret it as a set of polynomials divided by $(x^n-1)$:
$Z[X]/(x^n-1)=\{{p(x) \over x^n-1} : p(x) \in Z[X]\}$

If it is supposed to mean anything else, it is bad notation IMO and deserves a big red cross through it.
• Apr 27th 2012, 02:36 AM
mrproper
Re: Need help and confirmation about some notational symbols
This is from Wikipedia article about quotient rings.

Quotient ring - Wikipedia, the free encyclopedia

Quote:

Now consider the ring R[X] of polynomials in the variable X with real coefficients, and the ideal $I = (X^2 + 1)$ consisting of all multiples of the polynomial $X^2 + 1$. The quotient ring $R[X]/(X^2 + 1)$ is naturally isomorphic to the field of complex numbers C, with the class [X] playing the role of the imaginary unit i. The reason: we "forced" $X^2 + 1 = 0$, i.e. $X^2 = −1$, which is the defining property of i.
I'm reading and I'm reading and I'm reading and still cannot imagine how it all fits together. For example it seems that $X^4 = 1$ in the ring $Z[X]/(X^4-1)$ which seems to make sense in the context of the above citation. We "make" a ring which satisfies this property. But I cannot still imagine the cosets and their representatives...
• Apr 27th 2012, 01:31 PM
ILikeSerena
Re: Need help and confirmation about some notational symbols
I see.

Well, a coset would be a polynomial in Z[X] plus a multiple of $X^4-1$.
A representative is any polynomial, with perhaps a multiple of $X^4-1$ subtracted, so there are no powers of 4 anymore (for easier comparison).

$Z[X]/(X^4-1)=\{ \{ P(X) + k(X^4-1) : k \in Z \} : P(X) \in Z[X] \}$.
• Apr 27th 2012, 10:44 PM
mrproper
Re: Need help and confirmation about some notational symbols
Thanks very much! It's getting clearer now.
• Apr 28th 2012, 01:13 AM
ILikeSerena
Re: Need help and confirmation about some notational symbols
I just saw I need to modify my equation.
I've edited it in my previous post.

An element of $Z[X]/(X^4-1)$, which is a coset, is $\{ P(X) + k(X^4-1) : k \in Z \}$, where P(X) is a polynomial in Z[X].

So: $Z[X]/(X^4-1)=\{ \{ P(X) + k(X^4-1) : k \in Z \} : P(X) \in Z[X] \}$
• Apr 28th 2012, 02:46 PM
Deveno
Re: Need help and confirmation about some notational symbols
the notation (f(x)) is often used for the principal ideal generated by f(x), that is: all polynomials of the form k(x)f(x), for k(x) in Z[x].

note that the cosets p(x) + (f(x)) consist of the cosets of those polynomials p(x) where deg(p(x)) < deg(f(x)) (since we can use the division algorithm to write

p(x) = q(x)f(x) + r(x) in which case p(x) + (f(x)) = [r(x) + (f(x))] + [q(x)f(x) + (f(x))] = r(x) + (f(x)) since q(x)f(x) is in (f(x)) so that q(x)f(x) + (f(x)) = 0 + (f(x)) = (f(x)) itself).

the "classic example" is where f(x) = x2 + 1. note in this case, if we write I = (x2 + 1), in Z[x]/I we have cosets of the form:

a + bx + I, where a,b are integers. also note that for the coset x + I:

(x + I)(x + I) + (1 + I) = (x2 + I) + (1 + I) = (x2 + 1) + I = I, the 0-element of Z[x]/I.

thus a + bx + I ↔ a + bi is a (ring) isomorphism of Z[x]/I with Z[i], the gaussian integers (replacing "polynomials in x" with "polynomials in i" (i being a square root of -1)).