# Mathematical writing : using an “out-of-date” notation

When I wrote my master's thesis, a professor who read it said that I should not use the phrase "A function of class $k$." but instead "A function of class $C^k$". I am not an expert about mathematical history of notations, but I read that in Geometric Measure Theory, H. Federer actually uses the first one, and it seems logical for me: I think that $C^k$ is the abbreviation for "of class $k$". Therefore, employing "class $C^k$" seems like a repetition. Or maybe the other notation is just not used any more and should simply be prohibited?

• A google-books search using the word "function" and the phrase "of class $k$" will show you "class $k$" is used in a variety of settings. Notation conventions tend to come and go, but I'm willing to bet that "of class $k$" will be a lot less meaningful 50 years from now than "of class $C^{k}$". – Dave L Renfro Sep 15 '15 at 18:25
• I always thought of the $C$ as standing for "continuously-differentiable" – Eric Wofsey Sep 15 '15 at 18:25
• Federer could use a simplified notation in his book, if the term occurs very frequently. The standard notation is $C^k$. "Class $k$" will not be recognizable by most mathematicians. – Alexandre Eremenko Sep 15 '15 at 18:37
• Thank you Prof. Eremenko. The notation does not appear very often in Federer's book, but your second argument convinces me to use $C^k$. – Paul-Benjamin Sep 15 '15 at 18:47
• If "a function of class $C^k$" bothers you, you can always say "a $C^k$ function". – Noah Stein Sep 15 '15 at 20:49

• He's not the only one. Karl Menger invented a new notation and made consistent use of it and wrote much about it. Parts of it entered ordinary notation, parts didn't. Dirac vector covector notation is another example that however became popular, but whether a mathematician uses it depends on whether they feel like it, there are several standard options. Use the standard notation, unless your own notation is much better, in which case full speed ahead with your own and don't worry if somebody dislikes it. (If it's good others will like it too later.) Here AE is correct, $C^k$ is better. – Guido Jorg Sep 16 '15 at 18:33