This might be a really simple question, but I just didn't find an answer from anywhere.
I'm teaching linear algebra to myself and in my study material I came upon notation that I just don't understand. I can't find an explanation for it from my material and it is hard to find on the internet as well it seems.
Example:
$U = L((3,2,-6,4),(0,4,1,-5))$
What does the L() notation mean? U itself should be a subspace for ℝ⁴. I would assume that those are vectors within the L().
I agree with the commenters: it's most likely the linear span ($L$ for linear) of the vectors in parenthesis. But that is from context rather than from convention.
In general, the best place to look for the answer to a question like this is the materials you are studying from.
When you take a course, you get a textbook and/or lecture notes. Those texts are designed to be coherent and consistent. The definitions and notations may vary from text to text. Notation is calculus has pretty much been standardized, but less so in linear algebra.
When you're self-studying, I think you need to work in the same way. Concentrate on your one source. If there is a definition or notation you don't understand, work backwards until its first usage. There you'll find its meaning.