The concept of the Jacobson radical is quite new to me and I have to give a talk about a certain paper concerning it. I agreed to do this to find motivation to finally study this concept properly, as the paper was advertised to be accessible. Part of it is, but I ran into trouble when the author actually started using facts about the Jacobson radical. The paper is On the Jacobson Radical of Polynomial Rings by J. Krempa (1974).
Rings aren't necessarily unital here. An element $r$ of a ring $R$ is said to be quasi-regular when there is an element $s\in R$ such that $$r+s-rs=0.$$ (At least this seems to be the definition the author is using. Jacobson calls such elements right quasi-regular.) $R_n$ will denote the ring of $n\times n$-matrices over $R.$ A ring is nil iff all of its elements are nilpotent. A ring is radical iff it's equal to its Jacobson radical.
The aim of the paper is to prove the following statement.
Let $R$ be any ring. Then $R[x]$ is radical iff for all $n$ the ring $R_n$ is nil.
The proof starts with the following lemma.
For any ring $R$ and any $n\geq 1,$ there exists a monomorphism $f_n:R[[x]]\longrightarrow R_n[[x]]$ such that
(i) unimportant (ensuring that when $R$ is an algebra, the monomorphism respects the additional structure);
(ii) $f_n(R[x])=f_n(R[[x]])\cap R_n[x];$
(iii) If $p\in R[x]$ and $\deg p\leq k n,$ then $\deg f_n(p)\leq k.$
The proof of this lemma relies on two quite simple tricks. First, we send an element $p\in R[[x]]$ to an infinite matrix over $R$ like here. Then we "cut" the matrix into $n\times n$ blocks, obtaining an infinite matrix over $R_n.$ Then we notice that the resulting matrix is the image of a power series $r\in R_n[[x]],$ and we put $$f_n(p)=r.$$
The restriction of $f_n$ to $R[x]$ is a monomorphism to $R_n[x].$
Now, there's a second lemma, the one I'm having trouble with.
Let $R$ be any ring. Then if $R_n$ is a nil ring, any polynomial of degree $\leq n$ is quasi-regular in $R[x].$
The author takes any $p\in R[x]$ of degree $\leq n.$ He notes that $\deg f_n(p)\leq 1.$ And he says that $f_n(p)$ is quasi-regular in $R_n[x],$ which he doesn't explain and I don't understand. We have $f_n(p)=ax+b,$ where $a,b\in R_n[x]$ are nilpotent. I know that nilpotent elements are always quasi-regular, but in a noncommutative ring, the sum of two nilpotents doesn't have to be nilpotent. $(1)$ Why is $f_n(p)$ quasi-regular in $R_n[x]?$
The author denotes by $q$ the element of $R_n[x]$ such that $f_n(p)+q-f_n(p)q=0.$ Then the paper says this.
Since $R[[x]]$ is radical, there exists $p'\in R[[x]]$ such that $p+p'+pp'=0.$
$(2)$ Why is $R[[x]]$ radical? $(3)$ And why does it imply that $p$ is quasi-regular? From this Wikipedia page, I know that the Jacobson radical is the set of all elements $a$ of $R$ such that for any $r\in R,$ $ar$ is quasi-regular. So a radical ring is a ring in which any product of two elements is quasi-regular. But what about the elements which aren't products? There can be such elements in non-unital rings…
After that the author notes that $$f_n(p)+f_n(p')-f_n(p)f_n(p')=0,$$ and this is what follows:
The ring $R_n[[x]]$ also being radical, the element $f_n(p)$ has exactly one quasi-inverse. Therefore $q=f_n(p')\in f_n(R[x]),$ i.e. $p'\in R[x],$ since $f_n$ is a monomorphism of $R[x].$
I'm completely confused at this moment. I'm not even sure what the author means by "quasi-inverse". He seems to have changed the definition of "quasi-regular" to a one-sided version. $(4)$ Does it apply also to "quasi-inverse"?
Then it is said that from this lemma the non-obvious implication of the theorem follows.
EDIT Thanks to rschwieb's reference, I've been able to find answers to questions $(3)$ and $(4)$.
Best Answer
This is not really an explanation about the paper, but it is meant to help with the notion of quasiregularity here.
If you denote $r\circ s:=r+s-rs$, then you can show that $\circ$ is a binary operation on $R$ which is associative and has unity element $0$. (I read about this in Heatherly and Tucci's paper The circle semigroup of a ring, but I think Jacobson is usually credited for the idea.) If the ring has identity, then $1$ actually acts as an absorbing element! That is $a\circ 1=1\circ a=1$ for all $a\in R$. This is quite the reverse casting of 0 and 1 compared to the original operation!
Of course we can consider right and left ("quasi"-)inverses in this operation $\circ$. If you check, the definition you wrote for quasiregular was: $r\circ s=0$, that is, $s$ was a right inverse for $r$ (remember that 0 is the identity). As with any associative binary operation, if $r$ has any left inverse, then it has to be equal to $s$, but it is also possible that it has no left inverses.
Lam puts some exercises about this in First course in Noncommutative rings (and so you would also be able to see it in his Exercises in classical ring theory also)