Several good answers can be found on http://mathoverflow.net/q/95865 http://mathoverflow.net/q/95865 and http://math.stackexchange.com/q/45594/14018 http://math

Several good answers can be found on http://mathoverflow.net/q/95865 http://mathoverflow.net/q/95865 and http://math.stackexchange.com/q/45594/14018 http://math.stackexchange.com/q/45594/14018