Tadayoshi Takebayashi "Relations of the Weyl groups of extended affine root systems $A_1 ^{\left( {1,1} \right)} ,B_1 ^{\left( {1,1} \right)} ,C_1 ^{\left( {1,1} \right)} ,D_1 ^{\left( {1,1} \right)}$," Proceedings of the Japan Academy, Series A, Mathematical Sciences, Proc. Japan Acad. Ser. A Math. Sci. 71(6), 123-124, (1995)