Whereas I, not understanding the high school textbook or teacher explanations on some problem, but having reversed a procedure from the problems which had answe

Whereas I, not understanding the high school textbook or teacher explanations on some problem, but having reversed a procedure from the problems which had answers in the back of the book, was told one day: - It probably doesn't work for all problems in the set (but was given two chances to prove it from the unassigned problems and some the teacher made up on the spot). - Okay, but you can only use it on homework and still have to write every step; can't use it on quizes, tests, or extra credit because it takes up too much space and takes too long to grade. - If I could find a math symbol for the key operation within one day, I could use it ONE take-home extra credit that had been due that day. The key operation? For all positive N, f(N) = N+N-1...until N=zero. I couldn't find the requested symbo l. When I took my first uni maths class, I asked my prof. He answered and wrote it before I even finished describing: "Oh, sigma."