[1] Thomas J. Laffey, The Number of Solutions of $x^3 = 1$ in a 3-Group, Math. Z. 149, 43 -45 (1976). [2] THOMAS J. LAFFEY, The number of solutions of $x^p = 1$ in a finite group, Math. Proc. Camb. Phil. Soc. (1976), 80, 229. $J_3 \subset [0,\frac{7}{9}] \cup \{1\}$ follows from [1] and [2]; $J_2 \subset [0,\frac{3}{4}] \cup \{1\}$; it is mentioned in [1] that ``it is easy to check that if $G$ is a 2-group not of exponent 2, then $X_2(G)\leq \frac{3}{4}|G|$"; the case $G$ is not a 2-group follows from [2].