Σ is uppercase σ. Maybe this happened automatically? Pretty funny if so. Correct in a Greek context; clearly incorrect in a math context.

Yes, HN automatically did that.

For future reference, it is possible to edit the titles of stories you've submitted. This allows you to correct any errors introduced by HN's title rewriting heuristics at submission time, without waiting for a moderator to do it for you. Just like for comments, though, the edit window is time limited. For comments the window is two hours. I don't know if it's the same for story titles.