Yeah I think the FAQ is probably the best place to have a section about "open source". I'd reword the part about the maths as something like "Once you view software as useful math, an elaborate language, not like ordinary property, there is no reason to restrict others' use of it." For the bit about OpenBSD, it's probably best to add something like "(for network servers)". I don't know much about Bittorrent and if it's really that innovative, but it might be good to mention the protocol and the application.
I removed one mention of "open source" in the article (so now it's mentioned in points #1 and #5, each with a link to the footnote), the footnote now also points to our FAQ entry on "open source" (which is higher up in the FAQ page).
I re-formatted the list of examples, adding GNU/Linux next to OpenBSD and mentionned BitTorrent.
Finally I reworded the "maths" analogy as you suggested.
Thanks a lot!
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Force Content-Type helps.