MathOverflow is a question and answer site for professional mathematicians. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

Really sorry for this question, but googling for some time did not help me. I was trying to understand the meaning of the following phrase:

Let B be an augmented algebra over a semi-simple algebra T.

But I am stuck already with "augmented algebra"... -- can not find a definition on the web.

share|cite|improve this question
The notion of an augmented algebra is defined, for example, in Mac Lane's book Homology. There an augmented algebra $A$ over a field $k$ is a $k$-algebra that is also equipped with an algebra homomorphism $A \rightarrow k$ (the augmentation map). So in your context, I would assume that $B$ is a $T$-algebra that is also equipped with an algebra homomorphism $B \rightarrow T$. – Christopher Drupieski Feb 12 '12 at 0:16
It probably means that there is a map $\iota : T \to B$ of rings ($B$ is a $T$-algebra) which admits a splitting $\epsilon : B \to T$ (i.e. $\epsilon \circ \iota = 1_T$) – user91132 Feb 12 '12 at 0:18
up vote 11 down vote accepted

An augmented ring is simply a triple $(A,M,\epsilon)$ with $A$ ring, $M$ a left $A$-module and $\epsilon:A\to M$ a surjection of $A$ modules. One says then that $A$ is augmented over $M$. You can find this defined in Cartan-Eilenberg, for example.

Often, $M$ is itself a ring and the map $\epsilon$ is also a ring morphism. This is probably the situation you have.

share|cite|improve this answer
C-E also define supplemented algebras: a supplemented $k$-algebra is a $k$-algebra augmented over $k$ with $k$-linear augmentation. This is the concept that appears in the comments above. The name comes from the fact that in a supplemented algebra $A$ the base ring $k$ is a summand with a supplement, namely the kernel of the augmentation. – Mariano Suárez-Alvarez Feb 12 '12 at 0:23
Mariano, thanks a lot for a super quick answer! Just one more question. Why one would introduce the special name for such type of triples. Do they often pop up? – aglearner Feb 12 '12 at 0:24
I don't know who introduced the concept, but they surely come up often enough. In the context of Cartan-Eilenberg (it may well be they who came up with the notion) it serves as a unifying concept used to set up several different cohomology theories. (The supplemented algebra term is mostly unused nowadays, I'd say, but supplemented algebras are of course also all over the place) – Mariano Suárez-Alvarez Feb 12 '12 at 0:27
It appears they were indeed the inventors, at least according to Mac Lane classic review of the book (see…) «Another danger of shiny new notions is that sometimes the shine proves illusory. For example, the authors define an augmented ring [...] Hence "augmented ring" is a new name for "left ideal in a ring."» The review is a must read. – Mariano Suárez-Alvarez Feb 12 '12 at 0:31

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.