## What are examples of theorems which were once “valid”, then became “invalid” as standard definitions shifted?

That is, results established by correct proofs within some framework, yet the manner in which their author or the general mathematical community at the time would describe these results would, in later times, be interpreted as constituting a false claim, due to changing fashions as to how to standardly formalize some of the relevant concepts.

I imagine this sort of thing has happened often (e.g., with shifting accounts of "polyhedra" a la Lakatos' "Proofs and Refutations", or a motley of different definitions of "continuity" before standardization on the one we use now), but I do not have enough awareness of history to be able to provide solid examples (e.g., it seems plausible to me that Darboux may have considered himself to have proven that every derivative is continuous, taking the intermediate value property to be defining for continuity, but I do not know if this is an accurate account of what he claimed).

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This is similar to: mathoverflow.net/questions/31358 mathoverflow.net/questions/35468 – Bruce Westbury Jun 15 2011 at 18:11
Ah, thanks for those. I had already seen the first but it seemed a little different in focus than what I was after; I had not seen the second, which also seems a little different in that it encompasses out-and-out errors, but nonetheless seems to furnish some examples of what I was looking for. So I would not be opposed to closing/deleting this question, if it was felt the similarity was too great. – Sridhar Ramesh Jun 15 2011 at 18:23
Are you referring to shifting standards of rigor? For example, from Hilbert's point of view, Euclid's treatment was not fully rigorous, and from Zariski's point of view, some aspects of the Italian school of algebraic geometry were not rigorous. – Timothy Chow Jun 15 2011 at 21:55
How about this? "Every function can be represented by a Fourier series." Fourier seemed to think this was true, and it can be made to be true with suitable definitions of "function" and "represented," but not every such definition makes it true. Or what about computations with infinitesimals and with divergent series, which were considered O.K. at first, then not O.K., and then O.K. again, as people grappled with paradoxes and then eventually showed how to eliminate them with suitable definitions? – Timothy Chow Jun 16 2011 at 0:48
Hamming makes a big deal books.google.com/… of the fact that Hilbert did not disprove any of Euclid's theorems. – Allen Knutson Jun 16 2011 at 3:07

(This is basically a copy of my answer http://mathoverflow.net/questions/35468#35644 )

A prime example for a theorem that was considered "valid" but later became "invalid" is the following:

Theorem (Cauchy) Let $S_m(x) = \sum_{n=0}^m f_n(x)$ be the partial sums of a series on the interval $a \leq x \leq b$. If

1. $S_m(x)$ is continuous for all finite $m$
2. and $S_m(\xi)$ converges to $S(\xi)$ for all numbers $\xi$ in the interval

then the sum $S(x)$ is also continuous.$\square$

From the modern (Weierstraß) point of view, this theorem is wrong. A well-known counterexample is the trigonometric series ("sawtooth")

$$\sum_{k=1}^{\infty} \frac{\sin(kx)}k$$

which is not continuous at $x=0$.

However, this is not a counterexample to Cauchy's theorem as Cauchy understood it. His definitions of continuity and convergence were based on infinitesimals and the series violates condition 2. The point is that $\xi$ may be an infinitesimal.

In particular, let $n=\mu$ infinitely large and $\xi = \omega := \frac1\mu$ infinitesimally small. Then, the residual sum is

$$S(\omega) - S_{\mu-1}(\omega) = \sum_{k=\mu}^{\infty} \frac{\sin(k\omega)}k = \sum_{k=\mu}^{\infty} \frac{\sin(k\omega)}{k\omega}\omega \approx \int_{\omega\mu}^{\infty} \frac{\sin t}{t} \ dt = \int_1^{\infty} \frac{\sin t}{t} \ dt$$

Clearly, the integral is finite and not negligible; hence, the series does not converge for $\xi=\omega\approx 0$.

Put differently, condition 2 in Cauchy's sense is actually equivalent to uniform convergence. (I think)

I have taken this discussion and example from Detlev Laugwitz's paper "Definite values of infinite sums: Aspects of the foundations of infinitesimal analysis around 1820" (in particular pages 211-212).

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A fancier example is Kazhdan's proof of a conjecture of Langlands on conjugation of Shimura varieties. This appeared in the Budapest conference volume in the 1970s. Perhaps the sketchiness of the proof made comprehension of it non-trivial in the first place. In the year 1972 when I think the Budapest conference actually took place, it was not at all de rigeur to conform to Grothendieck (et al.)'s terminology and viewpoint, in part because that project had not been completed in all the aspects that might be convenient. Further, many people had grown up having to "improvise" an in-between viewpoint on algebraic geometry, especially rationality properties.

So, in short, perhaps that proof was correct in a certain context, became incorrect when the terminology was given extended sense under Grothendieck, but then became correct again when more things became known.

I believe Michael Harris and some collaborators may write up something about this sequence of events, or use it as an example.

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This sounds interesting, but your description is rather vague. Could you provide details? – Ryan Reich Jun 16 2011 at 20:57
If I recall correctly, Kazhdan proved that any galois conjugate of a compact Shimura variety is again a Shimura variety. Even nowadays, I think it is not so clear what conjugating a (projective) variety should mean, or might mean to someone else. One of the many things I could not understand was the tacit assumption that conjugation (whatever that meant) correspondingly conjugated (!?) the corresponding "Chow point" (whose meaning was in flux in those years, and is still not something of which I have any mastery...) Probably MathSciNet's review gives a better description of the theorem. – paul garrett Jun 16 2011 at 22:13