# Symbols in Linear Algebra [closed]

I have recently encountered a symbol I don't recognize which has to do with linear algebra.

It is $X^o$. What does this mean?

(And just to be sure, $X\leq V$ means $X$ is a subset of $V$, right? What is the difference does this have with $\subseteq$?)

Thanks

-
"I have recently encountered a symbol I don't recognize" - where? –  Ｊ. Ｍ. Nov 19 '11 at 11:18
I'll be very surprised if the book/article/website/other you're reading doesn't define this notation on a previous page. –  Chris Taylor Nov 19 '11 at 11:41

## closed as not a real question by Jonas Meyer, Asaf Karagila, t.b., Ｊ. Ｍ., Zev ChonolesDec 7 '11 at 4:56

It's difficult to tell what is being asked here. This question is ambiguous, vague, incomplete, overly broad, or rhetorical and cannot be reasonably answered in its current form. For help clarifying this question so that it can be reopened, visit the help center.If this question can be reworded to fit the rules in the help center, please edit the question.

$X \le V$, in the context of vector spaces, means that $X$ is a subspace of $V$, not just a subset.

$X^0$ probably means the annihilator of $X$: that is, the subspace of the dual $V*$ consisting of the linear functionals $f$ such that $f(x)=0$ for every $x$ in $X$.

-
I think, $X^0$ is the space $Y$ which satisfied that $y\in Y,yX=0$.
If $X$ is merely a subset of $V$, it could be the polar of $X$.