The align environment is used for two or more equations when vertical alignment is desired; usually binary relations such as equal signs are aligned.

American Mathematical Society, User's Guide for the amsmath Package

Introduction to align[edit | edit source]

For all intents and purposes, the align environment is a replacement for the eqnarray environment and all its warts. Rather than

\begin{eqnarray*}
x^2 + y^2 &=& 1 \\
y &=& \sqrt{1 - x^2},
\end{eqnarray*}

one can type

\begin{align*}
x^2 + y^2 &= 1 \\
y &= \sqrt{1 - x^2}.
\end{align*}

Benefits over eqnarray[edit | edit source]

Besides the slightly simpler syntax, you side-step bugs documented by Lars Madsen for The PracTeX Journal, such as

  • inconsistent spacing around binary symbols,
  • overwriting equation numbers, and
  • silent label mismatch.

Multiple equations on one line[edit | edit source]

Besides being used for aligning binary symbols, the ampersand can also mark an invisible alignment for separating columns of equations. For example,

\begin{align}
    u  &= \arctan x            &  dv &= 1 \, dx  \\ 
    du &= \frac{1}{1 + x^2}dx  &  v  &= x.
\end{align}

produces:

Preamble[edit | edit source]

To use align, import the amsmath package in your preamble.

.....
\usepackage{amsmath}
.....
\begin{align}
i_t & = \sigma(W_{xi}x_t+W_{hi}h_{t-1}+W_{ci}c_{t-1}+b_i)\\\\
f_t & = \sigma(W_{xf}x_t+W_{hf}h_{t-1}+W_{cf}c_{t-1}+b_f)\\\\
c_t & = f_t\odot c_{t-1}+i_t\odot tanh(W_{xc}x_t+W_{hc}h_{t-1}+b_c)\\\\
o_t & = \sigma(W_{xo}x_t+W_{ho}h_{t-1}+W_{co}c_{t}+b_o)\\\\
h_t & = o_t\odot tanh(c_t)\\\\
\end{align}
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