Notation for zero matrix
There is no universal standard notation for zero matrices or zero vectors. You are presumably asking within the context of a paper, book, or class --- you should figure out or ask about the convention used within this context.
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jmodest about 1 month
I'm confused by notation for matrices. Does $0_n$ mean a zero matrix of size nXn or a zero vector of size nX1 ?
hardmath about 5 yearsIt can be ambiguous without more context. Most authors will explain such a notation (briefly, at the beginning of the write-up or when first used). In most cases the context (which you've not referenced) can be used to resolve the author's intention if they've failed to be explicit.