# Define certain letter in math-mode to be always non-italic

Imagine I have a 500 page book about complex numbers.

But I always wrote just j in math-mode as the imaginary unit. But I should have written \mathrm{j}. Is there a command I could write into my preamble to substitute all j with \mathrm{j}? It would be quite some pain in the ass, to do it for all j manually. (There are no appearances of j as running index or other meanings)

For some greek letters where I have the same problem I used:

\let\Theta\varTheta


But I don't know how to do the same for "normal letters".

Minimal example:

\documentclass{article}
\usepackage{amsmath}

\begin{document}

$$j ~~is not~~ \mathrm{j}$$

\end{document}

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It seems similiar to this one. Can you use regular expression and grep for instance to solve your problem (without LaTeX)? – Hackbard_C Aug 4 '14 at 15:02

\documentclass{article}
\DeclareMathSymbol{j}{\mathalpha}{operators}{j}
$$j ~~is~~ \mathrm{j}$$
`