Abraham Robinson worked in applied mathematics for several decades. MathSciNet lists 12 articles by Robinson in wing theory. His production included the book


Robinson, A.; Laurmann, J. A. Wing theory. Cambridge, at the University Press, 1956.


The book is full of references to infinitesimals. Thus on page 36 one finds a reference to "infinitesimal horseshoe vortices of constant strength". Five years later Robinson published his first publication on infinitesimals in


Robinson, Abraham. Non-standard analysis. Nederl. Akad. Wetensch. Proc. Ser. A 64 = Indag. Math. 23 1961 432–440.


Are there any sources or references in the literature to a possible connection between Robinson's work exploiting infinitesimals in applied mathematics, on the one hand, and his eventual development of a rigorous mathematical theory thereof, on the other?


Note 1. I should add that Robinson's biographer J. Dauben was apparently unaware of the fact that Robinson's work in applied mathematics exploited "informal" infinitesimals extensively (at least I don't recall any mention of this in his book).



Is there a source linking Robinson's work in wing theory with his theory of infinitesimals?
It turns out that Robinson himself answered my question. On page 238 of his book "Non-standard analysis" ([Zbl 0151.00803](https://zbmath.org/0151.00803)), he explicitly connects infinitesimals in applied mathematics with hyperreal infinitesimals, in the following terms:



> 
> **9.4 Sources and doublets.** In the theory of continuous media, the
> language of infinitesimals has a long tradition, This tradition remained
> unbroken even though infinitesimals had been abandoned in pure Analysis.
> For example, the derivation of Euler's equations of motion is frequently
> based on the consideration of the balance of forces acting on a *very small*
> cube and several laws of Fluid Mechanics are expressed in terms of *fluid
> particles* whose dimensions are supposed to be infinitely small. Again, a
> doublet is defined by letting two sources of equal and opposite strength
> move infinitely close to each other while the strength of the sources varies
> in a suitable manner. It is usually taken for granted, and sometimes
> shown explicitly, that the use of infinitesimal language can be avoided.
> Naturally, this is not what we shall do here.
> 
> 
> 


Having discussed supersonic flow in terms of infinitesimals in chapter 9, Robinson comments on page 259:



> 
> The reader is referred to SCHLICHTING [1955] or THWAITES [1960] for
> information on boundary layer theory; and to WARD [1955] or ROBINSON
> and LAURMANN [1956] for an introduction to the theory of linearized
> supersonic flow.
> 
> 
> 


His 1956 book with Laurmann is the book "Wing theory" ([Zbl 0073.41901](https://zbmath.org/0073.41901)) mentioned above.


The conclusion is that there was a clear connection in Robinson's mind between his work in applied mathematics using infinitesimals, on the one hand, and the rigorous theory he developed a few years later.


