first, the formula number\begin{equation}
X (k) =\sum_{n=0}^{n1} x (n) e^{j \frac{2 \pi}{n} k n}=\sum_{n=0}^{n1} x (n) w_n^{kn},\quad k=0,1,..., N1.
\end{equation}
Next paragraph
(1)Add Begin{equation} and End{equation}, the formula will be automatically numbered;
(2)The formula will be empty one line after another paragraph, or the next paragraph will not automatically indent the beginning.
two, inline formulaInline formula: $ formula $ another line formula: $$ formula $$
three, the formula inside the space
Two x Quad spaces 
A \qquad b 

Width of two M 
Quad spaces 
A \quad b 

The width of a m 
Large space 
A\ b 

1/3m width 
Medium space 
A\;b 

2/7m width 
Small spaces 
A\,b 

1/6m width 
No spaces 
Ab 


Close 
A\!b 

Indent 1/6m Width 
three, long formula line
Using split: \begin{equation}
\begin{split}
F = \{f_{x} \in f_{c} &: ( s >  c) \\
&\quad \cap (\text{minpixels} <  s < \text{maxpixels}) \ \
&\quad \cap ( s_{\text{conected}} >  s \epsilon) \}
\end{split}
\end{equation}
where \ \ represents the aligned position of the newline,&.
four, inline plus symbolIf written as $\sum_{k=0}^{n1}$, the hyphen in the line will be written in the above and the following parameters as a corner label, should be written as $\sum\limits_{k=0}^{n1}$, when the inline symbol does not change.
Formula number advanced:
If you use the eqnarray environment , you can only display the numbers behind the above or the following equation, such as:
\begin{eqnarray}
a+b = 1 \ \
C+d = 2
The \end{eqnarray} number is displayed after the second equation.
Use the following statement to make the numbering appear in the middle of two equations:
\begin{equation}
\BEGIN{ARRAY}{L}
A+b=1 \ \
c+d=2
\end{array}
\end{equation}
Sometimes it is crowded between the following equations to avoid this situation by adding \displaystyle to each equation, such as:
\begin{equation}
\BEGIN{ARRAY}{L}
\displaystyle a+b=1 \ \
\displaystyle c+d=2
\end{array}
\end{equation}
The split environment can also be used to write numbers in the middle, such as:
\begin{equation}
\begin{split}
a+b=1\\
c+d=2
\end{split}
\end{equation}