First of all there are limeted number of symbols which can be used for subscription.
There are these symbols:
1 - '\u2081'
2-  '\u2082'
3-  '\u2083'
...
9 - '\u2089'
+ - '\u208A'
- - '\u208B'
= - '\u208C'
( - '\u208D'
) - '\u208E'