>>>>> What is the problem with @expr{C_(-1)}?
>>
>> I just replaced
>>
>> @expr{C_(-1)} with @expr{C_{(-1)}}
>>
>> , despite syntax checker's complaints, and run make pdf. And the 
>> resulting pdf looks the way it should look, with all of (-1) in the 
>> sub-index.
>
> Anyway, since @expr is just an alias for @math, I guess we need some TeX 
> expert to help us out here.  Anyone?
>

I have some knowledge of TeX, and ISTM that Vladimir is right: with C_(-1) 
only the opening parenthesis is subscripted, which means that you get 
"C₍-1)", whereas with C_{(-1)} the expression (-1) is subscripted, which 
means that you get "C₍₋₁₎".