Free alternatives to Zendesk Community Center?

Question

Software and Internet Services

Greetings!
What are the free equivalents of Zendesk Community Center?
Necessary features:
1. Creation of categories / subcategories
2. Creation of questions (delimitation of rights)
3. Commenting on questions (delimitation of rights)
4. Voting (+1 / -1) for questions / answers
5. Statistics of questions / answers by users and agents
6 A simple connection with an existing subdomain without the need to deploy some software on your side
7. Setting up notifications by filters, or simply having a mailing list (email) as such.
Thanks to!