Efficient Construction of U

[mlesnick]

Originally reported by: M L (GitHub: mlesnick)

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For every column operation performed on R in the initial computation of the RU-decomposition, the corresponding entry of U should be entered into the matrix constant time.

Is the code in fact doing these constant time updates? Whether the update is in fact being done in constant time depends on the details of how the row additions in the code are being performed.

todo: look into this and make the fix, if necessary.

[mlesnick]

Original comment by M L (GitHub: mlesnick):

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Planned updates to RIVET will likely eventually render this a moot point.

[mlesnick]

It turns out that the code is indeed doing this in constant time. Some minor optimizations seem possible, but not worth bothering with, especially since we might eventually incorporate Roy's vineyards code anyway.