Hi Xan,
I forwarded your 'complete' document to myself but count not
run it at all (my pgf is too old, I guess). At a guess, and
especially because you said it is urgent, you could try this
for the offending \index entry:
\index
[amplada+mitjana respecte de k+1 valors]
{amplada+mitjan respecte de \mathematics{k+1} valors}
Best wishes,
Taco
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