Question about the authorship of mathematical equation
If something has appeared, pretty much in any form, you need to cite it. If you don't you will always be open to a charge of plagiarism. 

I'll note that the proof is more important, in the grand scheme of mathematics, than the statement of it, so you don't lose anything by citing past work. 

Authorship is different from "ownership". You can't own mathematics, but you can copyright your own expression of it (or anything, basically).

But if you don't cite it and someone later asks "why didn't he/she know about this" then it is a negative for you. Always cite your predecessors. Celebrate their work as well as your own advances. 

Some of the intervening work has now been put on a firmer ground by your work, but that will only be obvious if you make the connection.