motivations for MOND/TeVeS

ERROR: problems during latex
INPUT:
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{
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\begin{math}\displaystyle \Del . (\mu \left(\frac{\Del\phi}{a_o}\right) \Del \phi) = 4\pi G\end{math}
}
\clearpage
{
\color{black}
\begin{math}\displaystyle \Del . \Del \phi = 4 \pi G\end{math}
}
\clearpage
\end{document}
STDERR:
This is pdfTeX, Version 3.141592653-2.6-1.40.24 (TeX Live 2022/Debian) (preloaded format=latex)
 restricted \write18 enabled.
entering extended mode
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LaTeX2e <2022-11-01> patch level 1
L3 programming layer <2023-01-16>
(/usr/share/texlive/texmf-dist/tex/latex/base/article.cls
Document Class: article 2022/07/02 v1.4n Standard LaTeX document class
(/usr/share/texlive/texmf-dist/tex/latex/base/fleqn.clo)
(/usr/share/texlive/texmf-dist/tex/latex/base/size12.clo))
(/usr/share/texlive/texmf-dist/tex/latex/amsmath/amsmath.sty
For additional information on amsmath, use the `?' option.
(/usr/share/texlive/texmf-dist/tex/latex/amsmath/amstext.sty
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(/usr/share/texlive/texmf-dist/tex/latex/amsmath/amsbsy.sty)
(/usr/share/texlive/texmf-dist/tex/latex/amsmath/amsopn.sty))
(/usr/share/texlive/texmf-dist/tex/latex/xcolor/xcolor.sty
(/usr/share/texlive/texmf-dist/tex/latex/graphics-cfg/color.cfg)
(/usr/share/texlive/texmf-dist/tex/latex/graphics-def/dvips.def)
(/usr/share/texlive/texmf-dist/tex/latex/graphics/mathcolor.ltx))
(/usr/share/texlive/texmf-dist/tex/latex/base/latexsym.sty)
(/usr/share/texlive/texmf-dist/tex/latex/l3backend/l3backend-dvips.def)
No file obAbqwVy75.aux.
(/usr/share/texlive/texmf-dist/tex/latex/base/ulasy.fd)
! Undefined control sequence.
l.17 \begin{math}\displaystyle \Del
                                    . (\mu \left(\frac{\Del\phi}{a_o}\right)...

! Undefined control sequence.
 \Del 
                \phi 
l.17 ...yle \Del . (\mu \left(\frac{\Del\phi}{a_o}
                                                  \right) \Del \phi) = 4\pi ...

! Undefined control sequence.
l.17 ...\mu \left(\frac{\Del\phi}{a_o}\right) \Del
                                                   \phi) = 4\pi G\end{math}
[1]
! Undefined control sequence.
l.22 \begin{math}\displaystyle \Del
                                    . \Del \phi = 4 \pi G\end{math}
! Undefined control sequence.
l.22 \begin{math}\displaystyle \Del . \Del
                                           \phi = 4 \pi G\end{math}
[2] (./obAbqwVy75.aux) )
(see the transcript file for additional information)
Output written on obAbqwVy75.dvi (2 pages, 888 bytes).
Transcript written on obAbqwVy75.log.