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<!-- #BeginEditable "Body" -->
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<h1 class="center">Gamma Function</h1>
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<p>The Gamma Function serves as a super powerful version of the <a href="factorial.html">factorial function</a>.</p>
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<p>Let us first look at the factorial function:</p>
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<div class="simple">
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<table style="border: 0; margin:auto;">
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<tbody>
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<tr>
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<td style="width:60px; text-align:center;"><img src="images/factorial.svg" alt="Factorial Symbol" height="117" width="26"></td>
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<td>
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<p>The <b>factorial function</b> (symbol: <b><font size="+1">!</font></b>) says to <b>multiply all whole numbers</b> from our chosen number down to 1.</p>
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<p>Examples:</p>
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<ul>
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<div class="bigul">
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<li><b>4!</b> = 4 × 3 × 2 × 1 = 24</li>
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<li><b>7!</b> = 7 × 6 × 5 × 4 × 3 × 2 × 1 = 5040</li>
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<li><b>1!</b> = 1</li>
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</div>
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</ul> </td>
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</tr>
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</tbody></table></div><br>
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<p>We can easily calculate a factorial from the previous one:</p>
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<p class="center"><img src="images/factorial-how.svg" alt="factorial multiply" height="61" width="279"></p>
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<p>As a table:</p>
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<div class="beach">
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<table align="center" width="400" border="0">
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<tbody>
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<tr style="text-align:center;">
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<th>n</th>
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<th>n!</th>
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<th> </th>
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<th> </th>
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</tr>
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<tr style="text-align:center;">
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<td>1</td>
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<td><b>1</b></td>
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<td>1</td>
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<td>1</td>
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</tr>
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<tr style="text-align:center;">
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<td>2</td>
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<td>2 × <b>1</b></td>
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<td>= 2 × <b>1!</b></td>
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<td>= 2</td>
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</tr>
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<tr style="text-align:center;">
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<td>3</td>
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<td>3 × <b>2 × 1</b></td>
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<td>= 3 × <b>2!</b></td>
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<td>= 6</td>
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</tr>
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<tr style="text-align:center;">
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<td>4</td>
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<td>4 × <b>3 × 2 × 1</b></td>
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<td>= 4 × <b>3!</b></td>
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<td>= 24</td>
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</tr>
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<tr style="text-align:center;">
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<td>5</td>
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<td>5 × <b>4 × 3 × 2 × 1</b></td>
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<td>= 5 × <b>4!</b></td>
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<td>= 120</td>
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</tr>
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<tr style="text-align:center;">
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<td>6</td>
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<td>etc</td>
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<td>etc</td>
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<td> </td>
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</tr>
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</tbody></table>
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</div><p>In fact we have this general rule:</p>
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<p class="center large">n! = n × (n−1)!</p>
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<p>Which says</p>
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<p class="center">"the factorial of any number is <b>that number</b> times<br>
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the <b>factorial of (that number minus 1)</b>"</p>
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<p>So 10! = 10 × 9!, ... and 125! = 125 × 124!, etc.</p>
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<p>Note: this is a "recurrence relation"</p>
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<h2>Beyond Whole Numbers</h2>
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<p>Can we have a function that works more generally? If so, what properties do we want?</p>
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<p>Firstly, it should "hit the mark" at each whole number:</p>
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<p class="center larger"><b>f(x) = x!</b> for whole numbers</p>
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<p>Here are some examples that do that:</p>
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<div class="center">
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<div class="boxa"><img src="images/graph-factorial-a.svg" alt="factorial graph a" height="374" width="182"><br>
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Straight Lines
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</div>
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<div class="boxa"><img src="images/graph-factorial-b.svg" alt="factorial graph b" height="374" width="182"><br>
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Anything Goes</div>
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<div class="boxa"><img src="images/graph-factorial-c.svg" alt="factorial graph c" height="374" width="182"><br>
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Smooth</div>
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</div>
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<p>Next, we want this to be true:</p>
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<p class="center larger"><b>f(</b><b><b>z</b>) =</b> <b><b>z</b></b> <b>f(</b><b><b>z</b></b><b>−1)</b> in between the whole numbers</p>
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<p class="center"><img src="images/graph-gamma-gap1.svg" alt="gamma graph gap=1" height="242" width="279"><br>
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<b>f(b) = b f(b−1)</b><br>
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Example: <b>f(2.62) =</b> <b><b>2.62</b> × f(</b><b><b>1.62</b>)</b></p>
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<p>We also need a special condition to keep it smooth called "logarithmically convex": the <a href="../sets/functions-composition.html">composition</a> of the <a href="../sets/function-logarithmic.html">logarithm</a> with our function should be <a href="../calculus/concave-up-down-convex.html">convex</a>.</p>
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<p>Many functions have been discovered with those properties. They each have good and bad points.</p>
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<p>The one most liked is called the Gamma Function (<b>Γ</b> is the Greek capital letter Gamma):</p>
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<div class="center large">Γ(z) =
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<div class="intgl">
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<div class="to">∞</div>
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<div class="symb"></div>
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<div class="from">0</div>
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</div> x<sup>z−1</sup> e<sup>−x</sup> dx</div>
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<!-- (z) = INT{0,INF} x^z-1 e^-x dx -->
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<p>It is a <a href="../calculus/integration-definite.html">definite integral</a> with limits from 0 to infinity.</p>
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<p>It matches the factorial function for whole numbers (but sadly we must subtract 1):</p>
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<div class="center large"><b>Γ(n) = (n−1)!</b> for whole numbers</div>
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<p>So:</p>
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<ul>
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<li>Γ(1) = 0!</li>
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<li>Γ(2) = 1!</li>
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<li>Γ(3) = 2!</li>
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<li>etc</li></ul>
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<p>Let's see how to use it.</p>
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<div class="dotpoint">
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<p>How about n=1</p><br>
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<div class="center large">Γ(1) =
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<div class="intgl">
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<div class="to">∞</div>
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<div class="symb"></div>
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<div class="from">0</div>
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</div> x<sup>1−1</sup> e<sup>−x</sup> dx</div>
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<div class="center large"> =
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<div class="intgl">
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<div class="to">∞</div>
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<div class="symb"></div>
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<div class="from">0</div>
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</div> x<sup>0</sup> e<sup>−x</sup> dx</div>
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<div class="center large">=
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<div class="intgl">
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<div class="to">∞</div>
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<div class="symb"></div>
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<div class="from">0</div>
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</div>e<sup>−x</sup> dx</div>
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<div class="center large">= <span class="limit"><em>lim</em><strong>x→∞</strong></span> (−e<sup>−x</sup>) − (−e<sup>−0</sup>)</div>
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<div class="center large">= 0 − (−1)</div>
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<div class="center large">= 1</div>
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</div> <p>Good so far, but does it work generally (<b>z</b> not restricted to integers)?</p>
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<div class="center large"><b>Γ(z+1) = z Γ(z)</b> </div>
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<div class="example">
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<p>Let us try:</p>
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<div class="center large">Γ(z+1) =
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<div class="intgl">
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<div class="to">∞</div>
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<div class="symb"></div>
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<div class="from">0</div>
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</div> x<sup>z+1−1</sup> e<sup>−x</sup> dx</div>
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<div class="center large"> =
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<div class="intgl">
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<div class="to">∞</div>
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<div class="symb"></div>
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<div class="from">0</div>
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</div> x<sup>z</sup> e<sup>−x</sup> dx</div>
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<p>We can use <a href="../calculus/integration-by-parts.html">Integration by Parts</a> with u=x<sup>z</sup> and v=e<sup>−x</sup>. There are many steps, but the key points are:</p>
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<div class="center large">Γ(z+1) = <span style="display:inline-block; transform: scaleY(2);">[</span> −x<sup>z</sup> e<sup>−x</sup> <span style="display:inline-block; transform: scaleY(2);">]</span>
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<div style="display:inline-block; font: 0.8em Verdana; text-align:center;transform: translateY(30%) translateX(-30%);">
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<div>∞</div><br>
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<div>0</div>
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</div>+<div class="intgl">
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<div class="to">∞</div>
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<div class="symb"></div>
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<div class="from">0</div>
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</div>zx<sup>z-1</sup> e<sup>−x</sup> dx</div>
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<div class="center large">Γ(z+1) = <span class="limit"><em>lim</em><strong>x→∞</strong></span>(−x<sup>z</sup> e<sup>−x</sup>) − (−0<sup>z</sup> e<sup>−0</sup>) + z
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<div class="intgl">
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<div class="to">∞</div>
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<div class="symb"></div>
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<div class="from">0</div>
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</div>x<sup>z-1</sup> e<sup>−x</sup> dx</div>
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<p>And −x<sup>z</sup> e<sup>−x</sup> goes to 0 as z goes to infinity, so it all simplifies to:</p>
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<div class="center large">Γ(z+1) = z
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<div class="intgl">
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<div class="to">∞</div>
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<div class="symb"></div>
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<div class="from">0</div>
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</div>x<sup>z-1</sup> e<sup>−x</sup> dx</div>
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<p>And the remaining integral is actually the Gamma Function for z, so:</p>
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<div class="center large">Γ(z+1) = z Γ(z)</div>
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<p>So it works generally.</p>
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</div>
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<p>And here is a plot of the Gamma Function:</p>
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<p class="center"><img src="images/gamma-plot.svg" alt="gamma function " height="400" width="600"></p>
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<p>But at x = 0 or less it works everywhere <b>except at integer values</b> because</p>
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<ul>
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<li><b style="white-space:nowrap;">Γ(0) = Γ(1)/0</b> is undefined (<a href="dividing-by-zero.html">dividing by zero</a>),</li>
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<li>and so Γ(−1) = Γ(0)/−1 is also undefined,</li>
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<li>etc <span style="font-family:Verdana,Arial,Tahoma,sans-serif"><span style="font-family:Verdana,Arial,Tahoma,sans-serif"><span style="font-family:Verdana,Arial,Tahoma,sans-serif"><b style="font-family:Verdana,Arial,Tahoma,sans-serif;text-align:center"><b style="font-family:Verdana,Arial,Tahoma,sans-serif;text-align:center"><font color="#2664b5"><span style="font-size:17.6px"><span></span></span></font></b></b></span></span></span></li></ul>
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<p><b>Try comparing</b> two values on the graph that are 1 apart on the x axis and see if it is true that Γ(z+1) = z Γ(z)</p>
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<h3>Complex</h3>
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<p>The Gamma Function also works for <a href="complex-numbers.html">Complex Numbers</a> so long as the real part is greater than 0.</p>
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<h3>Half</h3>
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<p>We can calculate the gamma function at <b>a half</b> (quite a few steps!) to get a surprising result:</p>
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<p class="center large">Γ(<span class="intbl"><em>1</em><strong>2</strong></span>) = <span class="times">√π</span></p>
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<p>Knowing that <b>Γ(z+1) = z Γ(z)</b> we get these "half-integer" factorials:</p>
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<div class="simple">
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<table style="border: 0; margin:auto;">
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<tbody>
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<tr>
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<td class="center">Gamma</td>
|
||
<td>Γ(z+1) = z Γ(z)</td>
|
||
<td><br>
|
||
</td>
|
||
<td>Factorial</td></tr>
|
||
<tr>
|
||
<td style="text-align:center; width:100px;">Γ(<span class="intbl"><em>1</em><strong>2</strong></span>)</td>
|
||
<td style="text-align:center;"><br>
|
||
</td>
|
||
<td style="text-align:center; width:100px;"><span class="times">√π</span></td>
|
||
<td style="text-align:center;">(−<span class="intbl"><em>1</em><strong>2</strong></span>)!</td>
|
||
</tr>
|
||
<tr>
|
||
<td style="text-align:center;">Γ(<span class="intbl"><em>3</em><strong>2</strong></span>)</td>
|
||
<td style="text-align:center;">= <span class="intbl"><em>1</em><strong>2</strong></span>Γ(<span class="intbl"><em>1</em><strong>2</strong></span>) =</td>
|
||
<td style="text-align:center;"><span class="intbl"><em>1</em><strong>2</strong></span><span class="times">√π</span></td>
|
||
<td style="text-align:center;">(<span class="intbl"><em>1</em><strong>2</strong></span>)!</td>
|
||
</tr>
|
||
<tr>
|
||
<td style="text-align:center;">Γ(<span class="intbl"><em>5</em><strong>2</strong></span>)</td>
|
||
<td style="text-align:center;">= <span class="intbl"><em>3</em><strong>2</strong></span>Γ(<span class="intbl"><em>3</em><strong>2</strong></span>) =</td>
|
||
<td style="text-align:center;"><span class="intbl"><em>3</em><strong>4</strong></span><span class="times">√π</span></td>
|
||
<td style="text-align:center;">(<span class="intbl"><em>3</em><strong>2</strong></span>)!</td>
|
||
</tr>
|
||
<tr>
|
||
<td style="text-align:center;">Γ(<span class="intbl"><em>7</em><strong>2</strong></span>)</td>
|
||
<td style="text-align:center;">= <span class="intbl"><em>5</em><strong>2</strong></span>Γ(<span class="intbl"><em>5</em><strong>2</strong></span>) =</td>
|
||
<td style="text-align:center;"><span class="intbl"><em>15</em><strong>8</strong></span><span class="times">√π</span></td>
|
||
<td style="text-align:center;">(<span class="intbl"><em>5</em><strong>2</strong></span>)!</td>
|
||
</tr><tr>
|
||
<td class="center">...</td>
|
||
<td class="center">...</td>
|
||
<td class="center">...</td>
|
||
<td class="center">...</td></tr>
|
||
</tbody></table>
|
||
</div><br>
|
||
<p>Also check if the graph above gets them right.</p>
|
||
|
||
|
||
<h2>Applications</h2>
|
||
|
||
<p>Just like the Factorial function there are many uses for the Gamma function in Combinatorics, Probability and Statistics. It is also very useful in Calculus and Physics.</p>
|
||
<p><br></p>
|
||
<p>So there you have it: the Gamma Function may be a little hard to
|
||
calculate but it neatly extends the factorial function beyond whole
|
||
numbers.</p>
|
||
|
||
<div class="related">
|
||
<a href="../combinatorics/combinations-permutations.html">Combinations and Permutations</a>
|
||
<a href="index.html">Numbers Index</a>
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