问：Is there a subjective map f: R2->S1xS1 that is regular everywhere?
答：yes, because for connected two-dimensional M there is always such a map f:R2->M. (Picture a long wide brushstroke.)
括号里的话该怎么理解呢？
答：yes, because for connected two-dimensional M there is always such a map f:R2->M. (Picture a long wide brushstroke.)
括号里的话该怎么理解呢？
