See this answer on the mathematics stack exchange (it's really a book excerpt, but I don't know where else to find it.) https://math.stackexchange.com/a/5390/22

See this answer on the mathematics stack exchange (it's really a book excerpt, but I don't know where else to find it.) https://math.stackexchange.com/a/5390/220797 https://math.stackexchange.com/a/5390/220797