tikz markers in a math equation of a node

Question

I realize this is almost certainly impossible; however, I would love to see the TeX wizardry hidden here (and I actually also really desperately want this feature).

So I have a tikzpicture, and inside this picture, I have some node. Inside this, node, I have a math equation, say:

    $$\left|
        \frac{
          \left(\sum_{i=1}^n a_i^2\right) \cdot
          \left(\sum_{i=1}^n b_i^2\right)
        }{ (\sum_{i=1}^n \underbrace{a_ib_i}_{HERE})^2} \right| \geq 1$

Now, the underbrace gives me a nice rotated "}", and I can label it "HERE". However, I don't want to label it "HERE".

I want to draw an arrow from a tikzNode to where the "HERE" is located. I want to be able to do something like:

    \draw [->] (cauchySchwarzLabelNode) -- (HERE);

So basically, I need to be able to "grab the location of where 'HERE' is in the equation" ... and somehow draw an arrow to it from tikz.

Is there any hack to make this possible?

Answer 1

Here's a solution using the \subnode command from the tikzmark TikZ library.

\documentclass{article}
%\url{http://tex.stackexchange.com/q/117393/86}
\usepackage{tikz}
\usetikzlibrary{tikzmark}

\begin{document}

\begin{tikzpicture}[remember picture]
\node {\(\displaystyle\left|
        \frac{
          \left(\sum_{i=1}^n a_i^2\right) \cdot
          \left(\sum_{i=1}^n b_i^2\right)
        }{ (\sum_{i=1}^n \underbrace{a_ib_i}_{\subnode{brace}{}})^2} \right| \geq 1\)};
\draw[->] (-3,-3) to[out=0,in=-90] (brace.north);
\end{tikzpicture}
\end{document}

I used brace.north because the subnode is actually the subscript of the underbrace, not the point of the underbrace itself. Note the remember picture on the containing environment.