On the Narrow $2$-Class Field Tower of Some Real Quadratic Number Fields, Part I: Ranks

Abstract: We determine some properties of the narrow 2-class field tower of those real quadratic number fields whose discriminants are not a sum of two squares and for which their 2-class groups are elementary of order $4$. Here in Part I, we determine precisely which of these fields have the 2-class groups of their narrow 2-Hilbert class fields of rank 2.