diff --git a/.obsidian/workspace.json b/.obsidian/workspace.json
index 289607d..03137b5 100644
--- a/.obsidian/workspace.json
+++ b/.obsidian/workspace.json
@@ -6,31 +6,39 @@
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             "type": "leaf",
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-                "file": "Concurrent Systems/notes/12 - Calculus of communicating system.md",
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                 "mode": "source",
                 "source": false
               },
               "icon": "lucide-file",
-              "title": "12 - Calculus of communicating system"
+              "title": "13 - Weak Bisimilarity"
+            }
+          }
+        ]
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+              "icon": "lucide-file-text",
+              "title": "class 13"
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@@ -90,7 +98,8 @@
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@@ -202,10 +211,12 @@
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+    "Concurrent Systems/notes/12 - Calculus of communicating system.md",
     "Concurrent Systems/notes/12b - CCS cose varie.md",
     "Concurrent Systems/notes/11 - LTSs and Bisimulation.md",
     "HCIW/slides/HCI in the car.pdf",
@@ -244,11 +255,9 @@
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diff --git a/Concurrent Systems/notes/13 - Weak Bisimilarity.md b/Concurrent Systems/notes/13 - Weak Bisimilarity.md
new file mode 100644
index 0000000..2adebed
--- /dev/null
+++ b/Concurrent Systems/notes/13 - Weak Bisimilarity.md	
@@ -0,0 +1,10 @@
+The equivalence studied up to now is quite discriminating, in the sense that it distinguishes, for example, τ.P and τ.τ.P.
+
+- If an external observer can count the number of non-observable actions (i.e., the τ’s), this distinction makes sense.
+- If we assume that an observer cannot access any internal information of the system, then this distinction is not acceptable.
+
+The idea of the new equivalence is to ignore (some) τ’s:
+- a visible action must be replied to with the same action, possibly together with some internal actions
+- an internal action must be replied to by a (possibly empty) sequence of internal actions.
+
+$P \implies P'$ if and only if there exist $P_{0}, P_{1},\dots,P_{k}$ (for $k \geq 0$) such that $P=P_{0} \xrightarrow{\tau} P_{1} \xrightarrow{\tau}$
\ No newline at end of file
diff --git a/Concurrent Systems/slides/class 13.pdf b/Concurrent Systems/slides/class 13.pdf
new file mode 100644
index 0000000..06e7062
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