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Post Closed as "Not suitable for this site" by YCor, Steven Landsburg, Dima Pasechnik, Pace Nielsen, Jan-Christoph Schlage-Puchta
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Why is the matrix of all 1's called "J"?

I've seen J referenced recently in some discussion about algebraic combinatorics, and it took me a while to figure out it was the matrix of all ones. It came up without definition, and I spent too long trying to google "what is a J matrix".

I found this, but I can find no reference to the convention of using J to represent the matrix. Maybe it's arbitrary?

https://en.wikipedia.org/wiki/Matrix_of_ones