From: Robin Chapman
Subject: Re: units of the ring A[X]
Date: Thu, 27 May 1999 13:41:45 GMT
Newsgroups: sci.math
In article <01bea81f$57e79580$37b939c2@buromath.ups-albi.fr>,
"pascal ORTIZ" wrote:
> Let A be a commutative ring with 1.
> The units (ie invertible elements) of the ring A[X] are of type
> a_0+a_1X+...+a_nX^n
> where a_0 is a unit in A and the a_k's (k>0) are nilpotent elements of A.
> This can be proved quickly by reducing the problem to the obvious case A
> where
> A is an integral domain and by use of the following well known result :
> if a is in P for all prime ideal P of A then a is nilpotent.
> I'm loocking for an elementary proof (avoiding the above result).
The difficult bit is to show that for a unit f(x) = a_0 + a_1 x + ...
+ a_n x^n then a_1, ..., a_n are nilpotent. Obviously a_0 is a unit
of A. As the sum of a nilpotent and a unit is a unit then all we
need is to show that a_n is nilpoent. Let the inverse of f(x)
be g(x) = 1 + b_1 x + ... + b_m x^m. Then a_n b_m = 0,
a_n b_{m-1} + a_{n-1} b_m = 0 so muliplying by a_n gives
a_n^2 b_{m-1} = 0. Keeping going eventually gives a_n^{m+1}b_0 = 0
and as b_0 is a unit, then a_n is nilpotent.
Robin Chapman + "They did not have proper
School of Mathematical Sciences - palms at home in Exeter."
University of Exeter, EX4 4QE, UK +
rjc@maths.exeter.ac.uk - Peter Carey,
http://www.maths.ex.ac.uk/~rjc/rjc.html + Oscar and Lucinda
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From: "A. Caranti"
Subject: Re: units of the ring A[X]
Date: Thu, 27 May 1999 16:14:32 +0200
Newsgroups: sci.math
pascal ORTIZ wrote:
> Let A be a commutative ring with 1.
> The units (ie invertible elements) of the ring A[X] are of type
> a_0+a_1X+...+a_nX^n
> where a_0 is a unit in A and the a_k's (k>0) are nilpotent elements of A.
> This can be proved quickly [...]
> I'm loocking for an elementary proof (avoiding the above result).
The following (sketch of a) proof is hopefully correct, and possibly
elementary.
First, it is immediate that for a_0 + a_1 X + ... to be invertible, a_0
must be a unit. So you are looking at elements of the form
1 - b X,
where b is another polynomial. If all coefficients of b are nilpotent,
then so is b, and 1 - b X is clearly invertibile by the usual formula
1 = (1 - b X) * (1 + b X + (b X)^2 + ...)
So now you want to go the other way: you have 1 - b X invertible, so
there is 1 + c X such that
1 = (1 - b X) * (1 + c X) = 1 + (c - b) X - b c X^2.
You get c = b, and thus b^2 = 0.
So you are left with proving that if a polynomial
b = b_0 + b_1 X + ...,
satisfies b^2 = 0, then all of its coefficients are nilpotent. It is
immediate that b_0^2 = 0. Therefore
(b - b_0)^3 = 0.
It follows that the polynomial
b_1 + b_2 X + ...
is nilpotent. Now take this as a model to set up an induction (b
nilpotent implies b_0 nilpotent and b_1 + b_2 X + ... nilpotent) and you
are done.
Andreas