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Number theory Difficulty 5.8 AIME, harder Prove it Turkey

Find all pairs (m,n)(m, n) of positive integers satisfying 2n+n=m!2^n + n = m!.

Solution

m=1m = 1 gives no solution, therefore m!m! is even and nn should also be even. Let n=2tsn = 2^t \cdot s (tt and ss are positive integers, t1t \ge 1 and ss is odd). t=1t = 1 readily leads to n=2n = 2 and m=3m = 3.

Now let t2t \ge 2. Then m!=2n+n=22ts+2ts22t+2tm! = 2^n + n = 2^{2t \cdot s} + 2^t \cdot s \ge 2^{2t} + 2^t. By induction over tt we will show that
22t+2t>(2t1)!(1) 2^{2t} + 2^t > (2t - 1)! \quad (1)
For t=2t=2, 33 the inequality holds. Assume that it is held for t=kt=k. In order to show that it also holds for k+1k+1 we have to prove that
22k+1+2k+122+2k2k(2k+1) \frac{2^{2k+1} + 2^{k+1}}{2^2 + 2^k} \ge 2k(2k+1)
Since 22n+2n2n+n2n1    22n2n(2n21)\frac{2^{2n} + 2n}{2^n + n} \ge 2^{n-1} \iff 2^{2n-2} \ge n(2^{n-2} - 1) and 22n2n2n22^{2n-2} \ge n \cdot 2^{n-2} for each positive integer nn we get 22n+2n2n+n2n1\frac{2^{2n} + 2n}{2^n + n} \ge 2^{n-1}. By taking n=2kn = 2^k we get 22k+1+2k+122+2k22k1\frac{2^{2^{k+1}} + 2^{k+1}}{2^2 + 2^k} \ge 2^{2^{k-1}}. Thus, in order to complete the proof we will show that 22k12k(2k+1)2^{2^{k-1}} \ge 2k(2k+1) for k3k \ge 3. For k=3,4k = 3, 4 it is held since 27>422^7 > 42 and 215>722^{15} > 72 and for k5k \ge 5 we have 22k124k=22k22k2k(2k+1)2^{2^{k-1}} \ge 2^{4k} = 2^{2k} \cdot 2^{2k} \ge 2k(2k+1). (1) is proved.

By (1) m!>(2t1)!m! > (2t-1)! and consequently m2tm \ge 2t. Since 2n+n=22ts+2ts=2t(22tst+s)2^n + n = 2^{2t \cdot s} + 2^t \cdot s = 2^t(2^{2t \cdot s-t} + s) and 2ts2t>t2^t \cdot s \ge 2^t > t we get that 22tst+β2^{2t \cdot s-t} + \beta is odd. Therefore 2t2^t divides the maximal even factor of m!m! and since m2tm \ge 2t we get
α=m2+m4+t+t2+t4+ \alpha = \left\lfloor \frac{m}{2} \right\rfloor + \left\lfloor \frac{m}{4} \right\rfloor + \dots \ge t + \left\lfloor \frac{t}{2} \right\rfloor + \left\lfloor \frac{t}{4} \right\rfloor + \dots
Contradiction since t2t \ge 2. Thus, the only solution is: (m,n)=(3,2)(m, n) = (3, 2).

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