diff --git a/Concurrent Systems/notes/4 - Semaphores.md b/Concurrent Systems/notes/4 - Semaphores.md
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--- a/Concurrent Systems/notes/4 - Semaphores.md	
+++ b/Concurrent Systems/notes/4 - Semaphores.md	
@@ -1 +1,8 @@
-A Semaphore is an object with 
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+**Object:** entity with an implementation (hidden) and an interface (visible), made up of a set of operations and a specification of the behavior.
+**Concurrent:** if the object can be accessed by different processes.
+
+**Semaphore:** is a shared counter S accessed via primitives $up$ and $down$ s.t.:
+- is initialized at s0 >= 0
+- it is alwayz >= 0
+- up atomically increases S
+- down atomically decreases S if it is not 0, otherwise the proce
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