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<title>The GNU Fortran Compiler: ATAN</title>
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<a name="ATAN-_002d_002d_002d-Arctangent-function"></a>
<h3 class="section">8.21 <code>ATAN</code> &mdash; Arctangent function</h3>
<a name="index-ATAN"></a>
<a name="index-DATAN"></a>
<a name="index-trigonometric-function_002c-tangent_002c-inverse"></a>
<a name="index-tangent_002c-inverse"></a>
<dl compact="compact">
<dt><em>Description</em>:</dt>
<dd><p><code>ATAN(X)</code> computes the arctangent of <var>X</var>.
</p>
</dd>
<dt><em>Standard</em>:</dt>
<dd><p>Fortran 77 and later, for a complex argument and for two arguments
Fortran 2008 or later
</p>
</dd>
<dt><em>Class</em>:</dt>
<dd><p>Elemental function
</p>
</dd>
<dt><em>Syntax</em>:</dt>
<dd><table>
<tr><td width="80%"><code>RESULT = ATAN(X)</code></td></tr>
<tr><td width="80%"><code>RESULT = ATAN(Y, X)</code></td></tr>
</table>
</dd>
<dt><em>Arguments</em>:</dt>
<dd><table>
<tr><td width="15%"><var>X</var></td><td width="70%">The type shall be <code>REAL</code> or <code>COMPLEX</code>;
if <var>Y</var> is present, <var>X</var> shall be REAL.</td></tr>
<tr><td width="15%"><var>Y</var> shall be of the same type and kind as <var>X</var>.</td></tr>
</table>
</dd>
<dt><em>Return value</em>:</dt>
<dd><p>The return value is of the same type and kind as <var>X</var>.
If <var>Y</var> is present, the result is identical to <code>ATAN2(Y,X)</code>.
Otherwise, it the arcus tangent of <var>X</var>, where the real part of
the result is in radians and lies in the range
<em>-\pi/2 \leq \Re \atan(x) \leq \pi/2</em>.
</p>
</dd>
<dt><em>Example</em>:</dt>
<dd><div class="smallexample">
<pre class="smallexample">program test_atan
real(8) :: x = 2.866_8
x = atan(x)
end program test_atan
</pre></div>
</dd>
<dt><em>Specific names</em>:</dt>
<dd><table>
<tr><td width="20%">Name</td><td width="20%">Argument</td><td width="20%">Return type</td><td width="25%">Standard</td></tr>
<tr><td width="20%"><code>ATAN(X)</code></td><td width="20%"><code>REAL(4) X</code></td><td width="20%"><code>REAL(4)</code></td><td width="25%">Fortran 77 and later</td></tr>
<tr><td width="20%"><code>DATAN(X)</code></td><td width="20%"><code>REAL(8) X</code></td><td width="20%"><code>REAL(8)</code></td><td width="25%">Fortran 77 and later</td></tr>
</table>
</dd>
<dt><em>See also</em>:</dt>
<dd><p>Inverse function: <a href="TAN.html#TAN">TAN</a>
</p>
</dd>
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