diff --git a/index.html b/index.html
index b4dbf51..2fbd37e 100644
--- a/index.html
+++ b/index.html
@@ -241,6 +241,8 @@ Error message and password prompt
 
 <li>$:/view</li>
 
+<li>arctanpi4</li>
+
 <li>Astronomia</li>
 
 <li>Fisica</li>
@@ -10548,7 +10550,7 @@ wikilist-button-foreground: #000</pre>
 <div created="20200728124256197" creator="u" modified="20200728124300028" modifier="ulaulama" title="$:/status/UserName">
 <pre>ulaulaman</pre>
 </div>
-<div created="20200803173126545" creator="ulaulaman" list="TikzDraw Menu" modified="20200803173806092" modifier="ulaulaman" title="$:/StoryList">
+<div created="20200803200854214" creator="ulaulaman" list="TikzDraw Matematica" modified="20200803200854214" modifier="ulaulaman" title="$:/StoryList">
 <pre></pre>
 </div>
 <div plugin-type="info" title="$:/temp/info-plugin" type="application/json">
@@ -10564,7 +10566,7 @@ wikilist-button-foreground: #000</pre>
         },
         &quot;$:/info/url/full&quot;: {
             &quot;title&quot;: &quot;$:/info/url/full&quot;,
-            &quot;text&quot;: &quot;file:///home/gianluigi/github/tikzdraw/empty.html&quot;
+            &quot;text&quot;: &quot;file:///home/gianluigi/github/tikzdraw/index.html&quot;
         },
         &quot;$:/info/url/host&quot;: {
             &quot;title&quot;: &quot;$:/info/url/host&quot;,
@@ -10584,7 +10586,7 @@ wikilist-button-foreground: #000</pre>
         },
         &quot;$:/info/url/pathname&quot;: {
             &quot;title&quot;: &quot;$:/info/url/pathname&quot;,
-            &quot;text&quot;: &quot;/home/gianluigi/github/tikzdraw/empty.html&quot;
+            &quot;text&quot;: &quot;/home/gianluigi/github/tikzdraw/index.html&quot;
         },
         &quot;$:/info/url/search&quot;: {
             &quot;title&quot;: &quot;$:/info/url/search&quot;,
@@ -11160,6 +11162,9 @@ For learning plugin features, syntax, tutorial and examples see the plugin demo
 <div created="20200728100805068" modified="20200728124217307" title="$:/view">
 <pre>classic</pre>
 </div>
+<div caption="arctan pi/4" created="20200803200958021" creator="ulaulaman" modified="20200803201154992" modifier="ulaulaman" tags="$:/tags/KaTeX/Snippet" title="arctanpi4">
+<pre>$$\arctan \frac{1}{2} + \arctan \frac{1}{3} = \frac{\pi}{4}$$</pre>
+</div>
 <div created="20200803173012723" creator="ulaulaman" modified="20200803173024722" modifier="ulaulaman" title="Astronomia" type="text/x-markdown">
 <pre>## Astronomy
 
@@ -11279,7 +11284,7 @@ See the [[control panel|$:/ControlPanel]] for more options.
 | *Masse* | [tex source](https://github.com/ulaulaman/tikzdraw/blob/master/infographics/sistema_solare/sistema_solare_masse.tex) | [pdf](https://github.com/ulaulaman/tikzdraw/blob/master/infographics/sistema_solare/pdf/sistema_solare_masse.pdf) |
 | *Densità* | [tex source](https://github.com/ulaulaman/tikzdraw/blob/master/infographics/sistema_solare/sistema_solare_densita.tex) | [pdf](https://github.com/ulaulaman/tikzdraw/blob/master/infographics/sistema_solare/pdf/sistema_solare_densita.pdf) |</pre>
 </div>
-<div created="20200803172912753" creator="ulaulaman" modified="20200803172920874" modifier="ulaulaman" title="Matematica" type="text/x-markdown">
+<div created="20200803172912753" creator="ulaulaman" modified="20200803201324992" modifier="ulaulaman" title="Matematica" type="text/x-markdown">
 <pre>## Mathematics
 
 | Title       | Source          | Pdf |
@@ -11296,7 +11301,9 @@ See the [[control panel|$:/ControlPanel]] for more options.
 | Square roots | [tex source](https://github.com/ulaulaman/tikzdraw/blob/master/math/square_roots.tex) | [pdf](https://github.com/ulaulaman/tikzdraw/blob/master/math/pdf/square_roots.pdf) |
 | Square of *n* | [tex source](https://github.com/ulaulaman/tikzdraw/blob/master/math/squares.tex) | [pdf](https://github.com/ulaulaman/tikzdraw/blob/master/math/pdf/squares.pdf) |
 | Trisecare un foglio di carta | [tex source](https://github.com/ulaulaman/tikzdraw/blob/master/math/trisecare_pagina.tex) | [pdf](https://github.com/ulaulaman/tikzdraw/blob/master/math/pdf/trisecare_pagina.pdf) |
-| Difference of two squares | [tex source](https://github.com/ulaulaman/tikzdraw/blob/master/math/differenze_quadrati.tex) | [pdf](https://github.com/ulaulaman/tikzdraw/blob/master/math/pdf/differenze_quadrati.pdf)</pre>
+| Difference of two squares | [tex source](https://github.com/ulaulaman/tikzdraw/blob/master/math/differenze_quadrati.tex) | [pdf](https://github.com/ulaulaman/tikzdraw/blob/master/math/pdf/differenze_quadrati.pdf) |
+| Arctan 1/2 + Arctan 1/3 = pi/4 | [tex source](https://github.com/ulaulaman/tikzdraw/blob/master/math/arctan-pi4.tex) | [pdf](https://github.com/ulaulaman/tikzdraw/blob/master/math/pdf/arctan-pi4.pdf) |
+</pre>
 </div>
 <div caption="Menu" created="20200731231208687" creator="ulaulaman" list-before="$:/core/ui/SideBar/Open" modified="20200803173817167" modifier="ulaulaman" tags="$:/tags/SideBar" title="Menu" type="text/vnd.tiddlywiki">
 <pre>| [[Infografiche]] |