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Non-commutative rings and algebras, non-associative algebras, universal algebra and lattice theory, linear algebra, semigroups. For questions specific to commutative algebra (that is, rings that are assumed both associative and commutative), rather use the tag ac.commutative-algebra.

9 votes
Accepted

Semiring naturally associated to any monoid?

It is at least sometimes called a "monoid semiring" by analogy with "group ring". As such it would be notated $S = \mathbb{N_0}[M]$ (or $\mathbb{N}[M]$ depending how you define things). By the way, …
Noah Stein's user avatar
  • 8,501
2 votes

Other Ring Structures on $\mathbb{Q}$

Since the question is somewhat fuzzy, I am not 100% sure what would satisfy you as an answer. Based on nothing, I am guessing you want something a little stranger than the examples Neil and Pace have …
Noah Stein's user avatar
  • 8,501
7 votes

Computer algebra systems that can handle real-closed fields?

Not really useful enough for an answer, but a little long to be a comment: While it is true that you can introduce extra variables and constrain their squares to be the quantities which you wish to b …
Noah Stein's user avatar
  • 8,501
10 votes
4 answers
1k views

Groups and rings which are not sets

An algebraic structure such as a group, ring, field, etc. is usually defined to be a set with some operations satisfying certain properties. I am curious what, if anything, goes wrong when the underl …
Noah Stein's user avatar
  • 8,501
11 votes
Accepted

Are the banded versions of a positive definite matrix positive definite?

No. The matrix $M = \begin{bmatrix}5 & 4 & 4 \\\\ 4 & 5 & 4 \\\\ 4 & 4 & 5\end{bmatrix} = \begin{bmatrix}2 & 2 & 2\end{bmatrix}\begin{bmatrix}2 \\\\ 2 \\\\ 2\end{bmatrix} + I$ is positive definite, …
Noah Stein's user avatar
  • 8,501