If a matrix is written with a double bar instead of square brackets, is there any significance?

matrices notation

Solution 1:

It looks like he means a matrix when he uses double bars:

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He uses the standard notation for a determinant, i.e. single bars:

enter image description here

I don't know if his notation for matrices was something that was common at the time he wrote the book, but I don't think it's popular now.

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