計量テンソル
•ポアンカレ円板
d s 2 = ( 4 / ( 1 − ( x 2 + y 2 ) ) 2 ) ( d x 2 + d y 2 ) {\displaystyle ds^{2}=(4/(1-(x^{2}+y^{2}))^{2})(dx^{2}+dy^{2})}