Cauchy proved a sum theorem for series of continuous functions in 1821, and published another article on the subject in 1853. 

Michael Segre, writing in *Archive for History of Exact Sciences*, claimed concerning Cauchy's sum theorem: 

*What is amazing here is Cauchy's attitude. He totally disregarded Fourier's counterexample and did not admit having made a mistake: not only did he "prove" his theorem, but he repeated it in a paper read to the Academie des Sciences as late as 1853.* (page 233 in [Segre, Michael. Peano's axioms in their historical context. Arch. Hist. Exact Sci. 48 (1994), no. 3-4, 201-342](http://link.springer.com/article/10.1007%2FBF00375085)) 

For his part, Umberto Bottazzini wrote: 

*The language of infinites and infinitesimals that Cauchy used here seemed ever more inadequate to treat the sophisticated and complex questions then being posed by analysis... The problems posed by the study of nature, such as those Fourier had faced, now reappeared everywhere in the most delicate questions of "pure" analysis and necessarily led to the elaboration of techniques of inquiry considerably more refined than those that had served French mathematicians at the beginning of the century. Infinitesimals were to disappear from mathematical practice in the face of Weierstrass' epsilon and delta notation* (p. 208 in Bottazzini, Umberto. The higher calculus: a history of real and complex analysis from Euler to Weierstrass. Translated from the Italian by Warren Van Egmond. Springer-Verlag, New York, 1986) 

These authors make Cauchy appear rather obstinate with regard to what is described by some historians as his famous "mistake". To a number of mathematicians who have studied Cauchy's work, such claims by historians seem surprising. Are we to accept them at face value? Is there more to the story than meets the eye?

Full disclosure: an analysis of this question by my coauthors and myself is presented in [this article](https://arxiv.org/abs/1704.07723). Additional relevant material is referenced [here](http://u.cs.biu.ac.il/~katzmik/infinitesimals.html). What I am seeking are other possible responses to this question from people who have examined Cauchy's writings. 

Note 1. I included in the article (on page 6) an extensive quotation from Cauchy that includes in particular his improbable substitution of $x=\frac{1}{n}$ in the remainder term $r_n$; see (new version of) article linked above. To a mathematician trained in the Weierstrassian framework this looks like a freshman calculus error. However, Robinson's framework enables an interpretation of this as evaluation at an infinitesimal point. Recall that the salient mathematical point here is that uniform convergence is expressible by a pointwise condition in the extended continuum. This is analogous to uniform continuity being expressible by a pointwise condition, namely S-continuity or *microcontinuity* (this last point is not strictly speaking related to the *sum theorem* but may help sort this out for those not closely familiar with the framework).

Note 2. For a related discussion of Cauchy see [this MSE post](https://math.stackexchange.com/q/455871/72694).