数学sub回忆 A group with order pq (p, q are distinct prime numbers). What's right about this group?[b]This group has only four subgroups up to isomorphism.[b]This group is commutative.[b]其他几个不记得了 dy/dx-y/t=t*e^-t. 问y（1）=1。问y/t在t趋向于无穷大时的极限 X,Y are topology. f is a continuous function from X to Y. what's the definitation for continuous?[b]应该选X上开集的映射在f的作用下仍是开集吧。。。 If R is a ring. Which one is a sub - ring of R.[b]选项有所有可逆的元素集; 所有元素a满足a^n=1等等。。。 1/(2*pi*i)乘以sinx^2/(x-1)^2的值。 ax+by+cz=d到原点的距离 T是linear transform.存在z,T^2(z)!=0，T^3(z)=0.问关于span(z,T(z),T^2(z))什么说法是正确的。 G上有一个运算"."满足对任意a和b，a.b.a=b，且存在单位圆。问以下哪些说法是正确的。[b]G是群[b]G上元素都可逆。先回忆这么多吧。。。考得很差，希望这些信息对后来的同学有用~