Euler is often credited with introducing the notation $f(x)$, & people cite the example $f(fracxa+c)$, where he had khổng lồ use parentheses around the function argument. On the other hand, when the argument was a single letter like $x$, I have mainly seen Johann Bernoulli and Euler just write $f, x$ or $fcolon x$ (or $phi, x$), without the parentheses. If I recall correctly even Lagrange in his lectures introduced the function notation without parentheses.

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Question: Did Euler (or Johann Bernoulli) ever write $f(x)$?

In case the answer is no, the follow up question is: when did it become standard lớn put parentheses around $x$?

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Rodrigo de Azevedo
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I’m guessing no. But how does one make sure? (Maybe with 85+ volumes of clean pdfs...)

Cajori, who started that $f(frac xa+c)$ example, points out a $varphi(z)$ in D’Alembert (1754, p 50).

For “standard”, I would say Lacroix (1797, p. 87):

4. Pour représenter une fonction sans indiquer, en aucune manière bình luận elle peut être composée, je me servirai de la caractéristique $mathrm f$; et il faudra entendre, par l"expression $mathrm f(x)$, une fonction quelconque de $x$, en comprenant sous cette dénomination tout ce que comporte la définition du mot fonction (Intr. nº 1) : on doit donc bien se garder de prendre la lettre $mathrm f$ pour un coefficient de $x$. J’indiquerai la substitution de $x+k$ aulieu de $x$ dans $mathrm f(x)$, en écrivant $mathrm f(x+k)$, et cela voudra dire que le résultat est composé en $x+k$, comme la fonction primitive l’est en $x$.

Side remark tying into your other question: This book of Lacroix writes “the function $f$” very often; e.g. Pp. 93, 212, 258, 483–496, 502, mainly when describing results of Monge who also did this a lot (but avoided unnecessary parentheses). I think “$f$” all started with solutions of PDEs depending on “arbitrary functions” — though only Dedekind, I would say, made them “objects” in the sense you want at the other question.

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Edit:In E213 “Remarques sur les mémoires précedens de M. Bernoulli” (1755), just quoted elsewhere, you can see Euler “forget” his evaluation colon and slip into writing $Phi"(x)$ (p. 215) and eventually $Phi(x)$ (p. 216). Same thing in E441 (1773, p 429). So in the end, yes.