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As seen on the reference URL, the texvc extension does not like the national
danish characters.

This works: <math>E_{x,y} = \frac{\% \mbox{ vare } x}{\% \mbox{ change in }
y}</math>

This doesn't: <math>E_{x,y} = \frac{\% \mbox{ være } x}{\% \mbox{ change in }
y}</math>

The only difference above is that a has been exchanged with æ.