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I asked this question already here, someone suggested to move the discussion to this website. I hope some of the Italian users (at least) can help me out with this curiosity.

I have always been used to the Italian pronunciation "de ics" of the differential dx. In principle there would be nothing wrong in saying "di ics", as the letter "d" is pronunced "di", but then why do Italian mathematicians say "de ics"?

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    Welcome to Italian.SE! – Charo Aug 26 '19 at 20:36
  • @Charo Thank you. – Gibbs Aug 26 '19 at 20:39
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    Qualcuno asserisce che la “d corsiva” della derivata parziale è in realtà la forma minuscola corsiva della corrispondente lettera cirillica, д, che in russo si legge appunto “de”. E di lì l'uso sarebbe passato indebitamente anche alle d di dx. Chissà. – DaG Aug 26 '19 at 22:34
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    I've heard both; personally I pronounce it “di”. – egreg Aug 27 '19 at 12:57
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    I have deleted my answer because it was based on old memories and I am starting to second-guessing myself about what I exactly remember... – Denis Nardin Aug 27 '19 at 17:18
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Three main differential calculus notations exists:

  • Newton's, favored in physics
  • Leibniz's, favored in multidimensional maths
  • Lagrange', favored in maths on a single dimension

Others exist but they are less used (https://en.m.wikipedia.org/wiki/Notation_for_differentiation)

The notation you mention is Leibniz's notation, which favored publishing philosophy and maths works in Latin and French (https://history.stackexchange.com/a/45828/4245). The notation itself has been published and explained by Leibniz mainly in Latin, which would not read dx as "de ics".

Far from being a reliable answer, I guess dx could be pronounced as in French "dè ics" because that's the original language of the earliest widespread text book around the subject: de l'Hôpital's Analyse des Infiniment Petits pour l'Intelligence des Lignes Courbes

I think that for whoever speaks French (even as a foreign language as in my case) it comes natural to read it that way.

The book was fairly successful and had great resonance throughout the mathematical community.

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