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3

These are the relevant lines in fontmath.ltx, the file that contains the basic math settings for LaTeX: \DeclareSymbolFont{symbols}{OMS}{cmsy}{m}{n} \DeclareMathSymbol{\Re}{\mathord}{symbols}{"3C} \DeclareMathSymbol{\Im}{\mathord}{symbols}{"3D} This is the corresponding part in plain.tex: \font\tensy=cmsy10 % math symbols \font\sevensy=cmsy7 ...

8

\Re and \Im are defined this way in plain.tex by knuth: \mathchardef\Re="023C \mathchardef\Im="023D this means that they are "ordinary" characters (0), that they are found in the math symbol font (2 = cmsy), and they are located at (hex) positions 3C and 3D in that font. the definitions were initially adopted into amstex along with all other names of ...

2

It does not answer the question why but my answer shows how to get what you want to get. Option 1 (recommended) \documentclass[preview,border=12pt,12pt]{standalone}% change it back to your own document class \usepackage{physics} \begin{document} \! \begin{aligned} z &= a + b i\\ \Re(z) &= a\\ \Im{z} &= b \end{aligned} \end{document} ...

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