unicode as valid naming symbols

[Mark H Harris]

greetings, I would like to create a lamda as follows:

√ = lambda n: sqrt(n)


On my keyboard mapping the "problem" character is alt-v which produces 
the radical symbol. When trying to set the symbol as a name within the 
name-space gives a syntax error:

 >>> from math import sqrt
 >>>
 >>> √ = lambda n: sqrt(n)
SyntaxError: invalid character in identifier
 >>>
 >>>

however this works:

 >>>
 >>> λ = lambda n: sqrt(n)
 >>>
 >>> λ(2)
1.4142135623730951
 >>>

   The question is which unicode(s) are capable of being proper name 
characters, and which ones are off-limits, and why?


marcus