# Question on meaning of a symbol: long thin C

I don't know how to write it in $\LaTeX.$ It is a tall skinny bold C. This is the context: A set is defined by:

where $\complement\atop{\smash \scriptstyle i}$ is the thing I don't understand. The $i$ is actually directly underneath the weird $C$ in this case. Can anyone explain what this means?

-
Does it look like $$E_f = \{i:{\complement_i} L(x)\ne 0\}$$? (The $\complement$ is \complement.) – MJD Jul 3 '12 at 14:41
Yes! But without the square brackets (not sure why I added those). – 098765 Jul 3 '12 at 14:43
Though it's kind of bolder and taller but that could just be the book. – 098765 Jul 3 '12 at 14:44
Wait.. what would be the $i$th complement of a set then?! – user2468 Jul 3 '12 at 14:45
On page 6 it seems that $\complement_i L(x)$ denotes the $i$-th coefficient of the formal power series $L(x)$. – martini Jul 3 '12 at 15:06

What about the notation $[z^n]f(z)$ to denote the coefficient? – Frank Science Jul 3 '12 at 15:14
@Frank Science : I didn't know. But still, my comment applies ; perhaps people working with generating functions prefer the $\complement$ notation to the $[z^n]f(z)$ notation. – Patrick Da Silva Jul 3 '12 at 18:12