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History and philosophy of mathematics, biographies of mathematicians, mathematics education, recreational mathematics, communication of mathematics.

2 votes

Errors, oversights, and misunderstandings in mathematical research

There have been many false proofs of the Jacobian Conjecture, and it might be interesting to have a survey of the attempts and why they ultimately failed (i.e., what holes there were in the attempted …
8 votes

Extremely messy proofs

My favorite example is Élie Cartan's original classification of the simple Lie algebras over the real numbers, the so-called real forms. The calculations and case analyses that he does in the five ex …
13 votes
Accepted

Who came up with the Euler-Lagrange equation?

According to Giaquinta and Hildebrandt (Calculus of Variations I, p. 70): "Euler's differential equation was first stated by Euler in his Methodus inveniendi [2], Chapter 2, no. 21. Quite often, one s …
Robert Bryant's user avatar
45 votes

Riemann's formula for the metric in a normal neighborhood

Perhaps the simplest way to understand this formula is to think about how you would go about deriving it: Try to find the 'best' coordinates you can centered on a given point and see what doesn't cha …
Robert Bryant's user avatar
13 votes

Old books still used

Gaston Darboux' magnum opus Leçons sur la Théorie générale des Surfaces et les Applications géométriques du Calcul infinitésimal (first edition 1890, I think; there is a second edition dating from aro …
4 votes

Origin of Laguerre geometry?

The formulation of Laguerre geometry in terms of dual numbers is a decidedly 'synthetic' one, meant to exhibit how this set of transformations can be regarded as a different 'real form' of the well-kn …
Robert Bryant's user avatar
6 votes

References for the Poincaré-Cartan forms

You could try the introductory sections of our book, "Exterior Differential Systems and Euler-Lagrange Partial Differential Equations" (authors Bryant, Griffiths, Grossman), which is available at http …
Robert Bryant's user avatar
17 votes
Accepted

Where did Sophus Lie write the group commutator for two one parameter groups

Looking around on the internet, I found an English translation of Lie's 1891 paper Die Grundlagen für die Theorie der unendlichen kontinuierlichen Transformationsgruppen. I. (I.e., The foundations of …
Robert Bryant's user avatar
20 votes
Accepted

Emergence of the orthogonal group

Your quote about Cartan thinking of $B_n$ and $D_n$ as 'projective groups..." is actually Cartan describing the lowest dimensional homogeneous space of these groups (except, of course, for a few excep …
Robert Bryant's user avatar
45 votes
Accepted

Are there some other notions of "curvature" which measure how space curves?

in addition to these excellent examples of non-local curvature quantities and their extensions to the non-smooth setting (which I am not sure the OP was anticipating), I might add the 'original' non-l …