Discussions on a mathematical induction problem Yue Kwok Choy Question A sequence of real numbers {an} is defined as follows. a 0 a 1 1, a2 3 and a n 3 3a n 2 a n 1 2a n (a) Let , n = 0, 1, 2, …. b k a k 2 a k 1 a k , k = 0, 1, 2, …. b n 2b n 1 Prove that (b) for all n . a n 2n . Hence, or otherwise, deduce that "Proposed" solution (a) Let P(n) be the proposition : " b n 2b n 1 " . b0 a 2 a1 a 0 3 1 1 1 For P(1), b1 a 3 a 2 a 1 (3a 2 a 1 2a 0 ) a 2 a 1 (3 3 1 2 1) 3 1 2 b1 2 b 0 . Assume P(k) is true for some k , b k 2b k 1 that is , …. (1) …. (2) For P(k + 1), b k 1 a k 3 a k 2 a k 1 3a k 2 a k 1 2a k a k 2 a k 1 2a k 2 a k 1 a k 2b k P(k + 1) is true. By the Principle of Mathematical Induction, P(n) is true n (b) Let P(n) be the proposition : " a n 2 For P(0), a0 1 2 For P(1), a1 1 21 For P(2), a 2 1 22 n . ". 0 P(0) , P(1), P(2) are true. Assume P(k), P(k + 1) and P(k + 2) are true for some k k 1 ak 2 , a k1 2 , k a k 2 2 , that is, k 2 For P(k + 3) , a k 3 3a k 2 a k 1 2a k (Note: we cannot use (2) directly as there are subtractions) 3a k 2 a k 1 a k 2a k 1 a k 3b k 2a k 1 a k 32b k 1 2a k 1 a k , by (a) 34b k 2 2a k 1 a k , by (a) = …. 32 1 22 2 3 2k b0 2a k1 a k k k 1 k 2 k 3 22 1 2 k 3 P(k + 3) is true. By the Second Principle of Mathematical Induction, P(n) is true n {0}. 1 Discussions In the above "Proposed" solution, (1) The mathematical induction in (a) is wrong, as the inductive hypothesis ( b k 2b k 1 ) is not used in proving P(k + 1) part. Therefore either you forfeit the use of mathematical induction and prove directly as in Method 1 below or modify your mathematical induction as in Method 2 below. Method 1 b n a n 2 a n 1 a n 3a n 1 a n 2a n 1 a n 1 a n 2a n 1 a n a n 1 2b n 1 Method 2 Let P(n) be the proposition : " b n 2b n 1 " . b0 a 2 a1 a 0 3 1 1 1 For P(1), b1 a 3 a 2 a 1 (3a 2 a 1 2a 0 ) a 2 a 1 (3 3 1 2 1) 3 1 2 b1 2 b 0 . Assume P(k) is true for some k , that is , b k 2b k 1 …. (1) For P(k + 1), By (1), b k 2b k 1 a k 2 a k 1 a k 2a k 1 a k a k 1 2a k 2 2a k 1 2a k 22a k 1 2a k 3a k 1 3a k 2 a k 1 2a k a k 2 a k 1 23a k 1 a k 2a k 1 a k 1 a k 2a k 3 a k 2 a k 1 22a k 2 a k 1 a k a k 3 a k 2 a k 1 2a k 2 a k 1 a k b k 1 2b k P(k + 1) is true. By the Principle of Mathematical Induction, P(n) is true n . As can be seen, Method 1 is better than Method 2. (2) The Proposed solution in Part (b) is done better than part (a) but is not satisfactory: (i) The deduction in the P(k + 3) part as denoted by " = …. " is not wrong , but rather uncomfortable. You may better write " = …. (use deduction, hence induction)" to tell others that you know how to use induction here, but you are not writing for simplicity. (ii) P(k + 2) , that is , a k 2 2k 2 is not really used in the proof of P(k + 3) in the above. You may cut that part, but it is still all right to leave it there. You can in fact prove separately b n 2 n by mathematical induction (or deduction) first and the proof of part (b) can be rewritten as follows: 2 Let P(n) be the proposition : " a n 2 n " . For P(0), a 0 1 20 For P(1), a1 1 21 For P(2), a 2 1 22 P(0) , P(1), P(2) are true. Assume P(k) and P(k + 1) are true for some k ak 2 , k a k1 2 k 1 , that is, …. (2) For P(k + 3) , a k 3 3a k 2 a k 1 2a k 3a k 2 a k 1 a k 2a k 1 a k 3b k 2a k 1 a k 3 2 k 2 2 k 1 2 k 3 2k 4 2k 2k 8 2k 2 k 3 P(k + 3) is true. By the Second Principle of Mathematical Induction, P(n) is true n {0}. 3