See the footnote on page 1: “Hereafter, we let n and m refer to the number of non-compactified space and time dimensions, or more generally to the effective sp

See the footnote on page 1: “Hereafter, we let n and m refer to the number of non-compactified space and time dimensions, or more generally to the effective spacetime dimensionality that is relevant to the low-energy physics we will be discussing later.”