I don't understand this

[Tom Goldring]

To get the symbol "aleph null" typically used to denote the cardinality 
of the set of natural numbers, I can simply type the eight keystrokes 
\aleph_0 and it will show up in LyX correctly.

Now suppose I want the set whose members are all countable subsets of 
the real numbers. This is typically denoted as R_\aleph_0 (where by "R" 
I don't mean the ordinary letter capital R, I mean the symbol 
$\mathbb{R}$ that is used to denote the set of real numbers, which I can 
get in math mode from the "del" (inverted triangle) menu. In other 
words, it's "R" with subscript "aleph null".

Here's the part I don't understand. If I put in the "R" followed by 
_\aleph_0, the R and the aleph show up correctly, but the zero (the 
subscript of the aleph) shows up as a different character (I think it's 
the character that's used in formal logic to mean something like "is not 
a proof of").

Could someone please explain to me what is going on here? I can't make 
any sense out of it.