\RequirePackage{luatex85}
\documentclass[border=10pt]{standalone}
%% Compile svg by running:
%% pdflatex example.tex #OR:  lualatex --output-format=pdf example.tex
%% pdf2svg example.pdf example.svg

\usepackage{amsmath,amssymb}
\usepackage[all]{xy}
\begin{document}%
$$%
\xymatrix@C+2.5em@R+2.5em{
    &
    x^n
        \ar[ddr]_*{\begin{Bmatrix}n\\k\end{Bmatrix}}
        \ar@/_15pt/[ddl]_*{(-1)^{n-k}\begin{Bmatrix}n\\k\end{Bmatrix}}  
    & \\
    &  & \\
    x^{(n)}
        \ar[rr]^*{L(n,k)}
        \ar[uur]_*{\begin{bmatrix}n\\k\end{bmatrix}}
    & &
    (x)_{n}
        \ar@/^15pt/[ll]^*{(-1)^{n-k}L(n,k)}
        \ar@/_15pt/[uul]_*{(-1)^{n-k}\begin{bmatrix}n\\k\end{bmatrix}}
}%
$$%
\end{document}