math-operators
I want to use a slanted \sum symbol denoting a different meaning from summation. I searched it in the TSE but didn't find a satisfactory answer: @summer gave one with a too large size and @wipet gave one in PDFLaTex only. Then how do I get a slanted \sum symbol with a suitable size in XeLaTex?
Here's an implementation with \ooalign:
\documentclass{article}
\usepackage{amsmath}
\newcommand{\avsum}{\mathop{\mathpalette\avsuminner\relax}\displaylimits}
\makeatletter
\newcommand\avsuminner[2]{%
{\sbox0{$\m@th#1\sum$}%
\vphantom{\usebox0}%
\ooalign{%
\hidewidth
\smash{\vrule height\dimexpr\ht0+1pt\relax depth\dimexpr\dp0+1pt\relax}%
\hidewidth\cr
$\m@th#1\sum$\cr
}%
}%
}
\makeatother
\begin{document}
\[
\sum_{i=1}^{m} \avsum_{j=1}^{n} a_{ij}
\]
\[
\textstyle\sum_{i=1}^{m} \avsum_{j=1}^{n} a_{ij}
\]
\[
\scriptstyle\sum_{i=1}^{m} \avsum_{j=1}^{n} a_{ij}
\]
\[
\scriptscriptstyle\sum_{i=1}^{m} \avsum_{j=1}^{n} a_{ij}
\]
\end{document}
Another solution, also provided by the mathtools package, is to use its \smashoperator macro. In fact, by writing \smashoperator[r]{...}, one can arrange that the material overlaps only on the right, rather than on both sides. (Observe that the word "Note" does not overlap with the material below the summation sign.)
Note that I would also create a macro, called (say) \VaR, to typeset the string VaR with upright letters. And, do note that there's no point in encasing P(X)\cdot x in curly braces.
\documentclass{article}
\usepackage{mathtools}
\newcommand\VaR{\mathrm{VaR}}
\begin{document}
\begin{equation}
\text{Note:}\smashoperator[r]{\sum_{X\leq\VaR_\alpha(X)}} P(X)\cdot x
\end{equation}
\end{document}
Addendum to address the OP's follow-up question: You may achieve your typesetting objective by (a) switching to the basic, i.e., centered version of the \smashoperator directive and inserting a suitable "kern" at the start of the subscript material. I know it's not fruitful to argue about tastes, but in my opinion the result of right-shifting the subscript material is anything but good-looking.
\documentclass{article}
\usepackage{mathtools}
\newcommand\VaR{\mathrm{VaR}}
\begin{document}
\[
\text{Note:}\smashoperator{\sum_{\mkern70mu X\leq\VaR_\alpha(X)}} P(X)\cdot x
\]
\end{document}
Best Answer
You can use TikZ as suggested in https://tex.stackexchange.com/a/326144/4427