DeclareFontFamily{U}{mathx}{hyphencharfont45}
DeclareFontShape{U}{mathx}{m}{n}{
      <5> <6> <7> <8> <9> <10>
      <10.95> <12> <14.4> <17.28> <20.74> <24.88>
      mathx10
      }{}
DeclareSymbolFont{mathx}{U}{mathx}{m}{n}
DeclareFontSubstitution{U}{mathx}{m}{n}
DeclareMathAccent{widecheck}{0}{mathx}{"71}

This is METAFONT, Version 2.718281 (TeX Live 2013/Debian)

kpathsea: Running mktexmf mathx10

! I can't find file `mathx10'. <*> ...=ljfour; mag:=1; nonstopmode; input mathx10 Please type another input file name ! Emergency stop. <*> ...=ljfour; mag:=1; nonstopmode; input mathx10 Transcript written on mfput.log.

grep: mathx10.log: File o directory non esistente

mktextfm: `mf-nowin -progname=mf mode:=ljfour; mag:=1; nonstopmode; input mathx10' failed to make mathx10.tfm. kpathsea: Running mktextfm mathx10

mktextfm: Running mf-nowin -progname=mf mode:=ljfour; mag:=1; nonstopmode; input mathx10

! Font U/mathx/m/n/10.95=mathx10 at 10.95pt not loadable: Metric (TFM) file not
found.
<to be read again>
relax
l.236 begin{equation}
I wasn't able to read the size data for this font,
so I will ignore the font specification.
[Wizards can fix TFM files using TFtoPL/PLtoTF.]
You might try inserting a different font spec;
e.g., type `Ifont<same font id>=<substitute font name>'.