A DESCRIPTION OF THE PROBLEM :
The Unicode superscript 2 character (², U+00B2) is classified as a "Number" from what I can tell. As such should it not be possible to have it be part of a Java identifier? It would make it a bit nicer to write code like:

  double cos²_a = Math.pow(Math.cos(a), 2);

For long, complex formulas it certainly improves readability. Sadly the latest Java compiler indicates this is an invalid character:

error: illegal character: '\u00b2'

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Compile source code with a local identifier including the U+00B2 character.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
As long as it is not the leading character it should be considered part of a Java identifier.
ACTUAL -
The compiler errors out.

CUSTOMER SUBMITTED WORKAROUND :
Use "2" in identifiers.