Review of Proofs that Really Count: The Art of Combinatorial Proof by Arthur T. Benjamin and Jennifer J. Quinn; MAA, 2003
Review 2 of Proofs that Really Count: The Art of Combinatorial Proof Author: Arthur T. Benjamin and Jennifer J. Quinn Publisher: MAA, 2003 $43.95, Hardcover Reviewer: William Gasarch Abbott has been teaching Costello combinatorics. Abbott: Costello, how many subsets are there of {1, . . . , n}? Costello: Oh. You can either choose 0 elements, or choose 1 element, or choose 2 elements, etc. So the answer is n n . i=0 i Abbott: Well . . . let me show you a di Âerent way to do it. The number 1 is either in the set A or not, so thats 2 choices. Then the number 2 is either in the set A or not, so thats 2 choices, etc. So the Ânal answer is 2 2 = 2n . So, Costello, you did the problem your way, I did it my way, and we got di Âerent answers. What can you conclude? Costello: That one of us is wrong? Abbott: No. We Âve shown. n n = 2n . i=0 i Costello: Really! I don Ât believe that! Prove it!! Abbott: We did! Costello: When? Abbott: Just now. Costello: What!? Abbott: Whats on Second. Costello: Who? Abbott: Who Âs on Ârst. Costello: (Ignoring reference) Usually when I do a math problem two ways and get two answers I assume one of them is wrong and try to Ând my error. Its better than what a friend of mine did in elementary algebra  do a problem three times and then take the average. Abbott: In math you can sometimes prove that two things are the same by solving the same problem two di Âerent ways. Costello: No way! Abbott: Way! Costello: I Âd like to read more about this. Do you have a book to recommend? Abbott: Yes. You should read Proofs that Really Count: The Art of Combinatorial Proof by Arthur T. Benjamin and Jennifer J. Quinn. Costello: Can you give me some examples from the book? Abbott: Okay. Here is one involving Fibonacci Numbers. Recall that the Fibonaci numbers are de Âned by f0 = 0, f1 = 1, and ( Ân Â¥ 2)[fn = fn Â1 + fn Â2 ]. I...