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Questions designed to generate a "big list" of certain results, examples, conjectures, etc. via many individual answers, each contributing one or a few instances. Such a question should typically be in Community Wiki mode (CW); after asking, please, flag for moderators attention requesting the question to be made CW.

19 votes

Interesting applications (in pure mathematics) of first-year calculus

An interesting application of calculus is the elementary polynomial case of Mason's ABC theorem. This yields, for instance, a completely trivial proof of the polynomial case of FLT (Fermat's Last Theo …
Bill Dubuque's user avatar
  • 4,736
20 votes

Slick ways to make annoying verifications

Proofs exploiting universality often provide nice examples of slick ways to avoid annoying special cases. For example, the matrix identities below have trivial algebraic proofs by proceeding "generica …
53 votes
Accepted

Direct proof of irrationality?

Below is a simple direct proof that I found as a teenager: THEOREM $\;\rm r = \sqrt{n}\;$ is integral if rational, for $\;\rm n\in\mathbb{N}$. Proof: $\;\rm r = a/b,\;\; {\text gcd}(a,b) = 1 \implies …
Bill Dubuque's user avatar
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6 votes

Good papers/books/essays about the thought process behind mathematical research

Franz Lemmermeyer's paper [1] contains a very interesting account of the truth about how Kummer was led to the invention of his ideal numbers (the popular legend is far from reality). In particular he …
2 votes

Can a mathematical definition be wrong?

This is a slightly different kind of example, namely, one where the original definition had to be revised when it later was realized that it was useless in certain contexts. Probably most readers have …
19 votes

Can a mathematical definition be wrong?

An important historical example is the difficult evolution of the correct definition of "integer" in algebraic extensions, i.e. defining algebraic integers. It was only with great difficulty that Dede …
5 votes

Are there any good nonconstructive "existential metatheorems"?

Jacobson's theorem that X^m = X rings are commutative provides an enlightening example, see my post below for further discussion and references Abstract Thought vs Calculation
4 votes

Examples of undergraduate mathematics separation from what mathematicians should know

One might argue that (exotic) counterexamples fall into the first category. For example, when I took Munkres' course in topology it was organized around many very carefully chosen (counter)examples sh …
14 votes

Counterexamples in algebra?

Harry Hutchins "Examples of commutative rings" may be of interest. It is based on his 1978 Chicago Ph.D. thesis under Kaplansky, and not surprisingly it serves as a useful complement to Kaplansky's …
7 votes

Books you would like to see translated into English

The following wonderful 54 page survey by O. Neumann on Kronecker's divisor theory could easily be turned into a book and would fill a very large gap in the English literature on such. I'm interested …
17 votes

Abstract thought vs calculation

Some of the prettiest examples of Dedekind's structuralism arise from revisiting proofs in elementary number theory from a highbrow viewpoint, e.g. by reformulating them after noticing hidden structur …
28 votes

Abstract thought vs calculation

One striking example that comes to mind is Nathan Jacobson's proof that rings satisfying the identity $X^m = X$ are commutative. This is model-theoretic and proceeds by a certain type of factorization …
29 votes

Papers that debunk common myths in the history of mathematics

One of the biggest myths in number theory is that work on Fermat's last theorem played a large role in the development of ideal theory and algebraic number theory. In fact it was much loftier goals su …