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- All vectors in this book are column vectors of dimension
$$n \times 1$$ , with$$n$$ being the number of elements in that vector. - A superscript is used for a sample’s index. The
$$i^{th}$$ element in the dataset is denoted$$x^{(i)}$$ . - A subscript is used for dimension.
$$x_1$$ of an$$n \times n$$ matrix denotes its first row.