$x$

$x^2$

${10}^{1}8$

${10}^{18}$

$x_2$

$x_{1}0$

$x_{10}$

$\sqrt{x}$

$\sqrt[n]{x}$

$\le$

$\ge$

$\pm$

$\mp$

$\sigma (n)$

$\mathrm{\Sigma }$

$\prod$

$\underset{x}{y}$

$\stackrel{x}{y}$

$\underset{i=1}{\stackrel{n}{\mathrm{\Sigma }}}$

$\overrightarrow{x}$

$\frac{x}{y}$

$(\frac{x}{y})$

$\left(\frac{x}{y}\right)$

$\lfloor \frac{x}{y}\rfloor$

$\lceil \frac{x}{y}\rceil$

$|x|=\{\begin{array}{ll}x& x\ge 0\\ -x& x<0\end{array}$

$1,2,\dots ,n$

$\cdots ,\dots ,⋮$

$a⋮b$

$S\subset T$

$S\subseteq T$

$x\times y$

$x÷y$

$x\mathrm{mod}y$

$x\equiv y(\mathrm{mod}p)$

$x(w)=\{\begin{array}{l}0,w=n\\ 1,w=s\end{array}$

$x$

$x$

$\underset{x=1}{\stackrel{n}{\mathrm{\Sigma }}}\underset{y=x+1}{\stackrel{n}{\mathrm{\Sigma }}}\sqrt{x+y}$

$\frac{1}{2}+\frac{1}{3}+\frac{1}{4}\dots +\frac{1}{n}$

$F(x)=\{\begin{array}{ll}1& ,x=1\\ 1& ,x=2\\ F(x-1)+F(x-2)& ,x\ge 2\\ \supset \end{array}$

$\underset{i=1}{\stackrel{n}{\mathrm{\Sigma }}}\left(\right)$