On Wed, 17 Jun 2015, Procházka Lukáš Ing. - Pontex s. r. o. wrote:
Hello,
thanks for the solution: \define\dif{\mathop{}\!\mfunction{d}}
Another alternative is (untested) \definemathcommand[dif][nolop]{\mfunction{d}\kern\zeropoint}
Some explanation for \mathop and \mfunction would be appreciated, and also explanation why \definemathcommand[dif][nolop]{\mfunction{d}} didn't work...
Roughtly, the above exapnds to \define\dif{\mathop{\mfunction{d}}\nolimits} When the argument of \mathop is a single character (d, in this case), mathop centers the charater on the 'math-axis'. The rationale is that such a behavior is desirable when defining single letter math operators such as sum, int, bigcap, bigcup, etc. Adding \kern\zeropoint makes TeX think that the argument of \mathop is more than a single character, so it does not center d along the math-axis. Aditya