Olympiad Maths Prep

Library / /7 of 11

Number theory Difficulty 9.0 Shortlist Prove it IMO

Determine all triples (a,b,c)(a, b, c) of positive integers for which abca b - c, bcab c - a, and cabc a - b are powers of 22.

Explanation: A power of 22 is an integer of the form 2n2^{n}, where nn denotes some nonnegative integer.

Solutions — 2

Solution 1

It can easily be verified that these sixteen triples are as required. Now let (a,b,c)(a, b, c) be any triple with the desired property. If we would have a=1a = 1, then both bcb - c and cbc - b were powers of 22, which is impossible since their sum is zero; because of symmetry, this argument shows a,b,c2a, b, c \geqslant 2.

Case 1. Among a,ba, b, and cc there are at least two equal numbers.

Without loss of generality we may suppose that a=ba = b. Then a2ca^{2} - c and a(c1)a(c - 1) are powers of 22. The latter tells us that actually aa and c1c - 1 are powers of 22. So there are nonnegative integers α\alpha and γ\gamma with a=2αa = 2^{\alpha} and c=2γ+1c = 2^{\gamma} + 1. Since a2c=22α2γ1a^{2} - c = 2^{2\alpha} - 2^{\gamma} - 1 is a power of 22 and thus incongruent to 1-1 modulo 44, we must have γ1\gamma \leqslant 1. Moreover, each of the terms 22α22^{2\alpha} - 2 and 22α32^{2\alpha} - 3 can only be a power of 22 if α=1\alpha = 1. It follows that the triple (a,b,c)(a, b, c) is either (2,2,2)(2, 2, 2) or (2,2,3)(2, 2, 3).

Case 2. The numbers a,ba, b, and cc are distinct.

Due to symmetry we may suppose that
2a<b<c 2 \leqslant a < b < c
We are to prove that the triple (a,b,c)(a, b, c) is either (2,6,11)(2, 6, 11) or (3,5,7)(3, 5, 7). By our hypothesis, there exist three nonnegative integers α,β\alpha, \beta, and γ\gamma such that
bca=2α,acb=2β,andabc=2γ. \begin{align*} bc - a & = 2^{\alpha}, \\ ac - b & = 2^{\beta}, \\ \text{and}\quad ab - c & = 2^{\gamma}. \end{align*}
Evidently we have
α>β>γ. \alpha > \beta > \gamma.
Depending on how large aa is, we divide the argument into two further cases.

Case 2.1. a=2a = 2.

We first prove that γ=0\gamma = 0. Assume for the sake of contradiction that γ>0\gamma > 0. Then cc is even by (4) and, similarly, bb is even by (5) and (3). So the left-hand side of (2) is congruent to 22 modulo 44, which is only possible if bc=4bc = 4. As this contradicts (1), we have thereby shown that γ=0\gamma = 0, i.e., that c=2b1c = 2b - 1.

Now (3) yields 3b2=2β3b - 2 = 2^{\beta}. Due to b>2b > 2 this is only possible if β4\beta \geqslant 4. If β=4\beta = 4, then we get b=6b = 6 and c=261=11c = 2 \cdot 6 - 1 = 11, which is a solution. It remains to deal with the case β5\beta \geqslant 5. Now (2) implies
92α=9b(2b1)18=(3b2)(6b+1)16=2β(2β+1+5)16 9 \cdot 2^{\alpha} = 9b(2b - 1) - 18 = (3b - 2)(6b + 1) - 16 = 2^{\beta}(2^{\beta + 1} + 5) - 16
and by β5\beta \geqslant 5 the right-hand side is not divisible by 3232. Thus α4\alpha \leqslant 4 and we get a contradiction to (5).

Case 2.2. a3a \geqslant 3.

Pick an integer ϑ{1,+1}\vartheta \in \{-1, +1\} such that cϑc - \vartheta is not divisible by 44. Now
2α+ϑ2β=(bcaϑ2)+ϑ(cab)=(b+aϑ)(cϑ) 2^{\alpha} + \vartheta \cdot 2^{\beta} = (bc - a\vartheta^{2}) + \vartheta(ca - b) = (b + a\vartheta)(c - \vartheta)
is divisible by 2β2^{\beta} and, consequently, b+aϑb + a\vartheta is divisible by 2β12^{\beta - 1}. On the other hand, 2β=acb>(a1)c2c2^{\beta} = ac - b > (a - 1)c \geqslant 2c implies in view of (1) that aa and bb are smaller than 2β12^{\beta - 1}. All this is only possible if ϑ=1\vartheta = 1 and a+b=2β1a + b = 2^{\beta - 1}. Now (3) yields
acb=2(a+b) ac - b = 2(a + b)
whence 4b>a+3b=a(c1)ab4b > a + 3b = a(c - 1) \geqslant ab, which in turn yields a=3a = 3.

So (6) simplifies to c=b+2c = b + 2 and (2) tells us that b(b+2)3=(b1)(b+3)b(b + 2) - 3 = (b - 1)(b + 3) is a power of 22. Consequently, the factors b1b - 1 and b+3b + 3 are powers of 22 themselves. Since their difference is 44, this is only possible if b=5b = 5 and thus c=7c = 7. Thereby the solution is complete.

Solution 2

As in the beginning of the first solution, we observe that a,b,c2a, b, c \geqslant 2. Depending on the parities of a,ba, b, and cc we distinguish three cases.

Case 1. The numbers a,ba, b, and cc are even.

Let 2A,2B2^{A}, 2^{B}, and 2C2^{C} be the largest powers of 22 dividing a,ba, b, and cc respectively. We may assume without loss of generality that 1ABC1 \leqslant A \leqslant B \leqslant C. Now 2B2^{B} is the highest power of 22 dividing acbac - b, whence acb=2Bbac - b = 2^{B} \leqslant b. Similarly, we deduce bca=2Aabc - a = 2^{A} \leqslant a. Adding both estimates we get (a+b)c2(a+b)(a + b)c \leqslant 2(a + b), whence c2c \leqslant 2. So c=2c = 2 and thus A=B=C=1A = B = C = 1; moreover, we must have had equality throughout, i.e., a=2A=2a = 2^{A} = 2 and b=2B=2b = 2^{B} = 2. We have thereby found the solution (a,b,c)=(2,2,2)(a, b, c) = (2, 2, 2).

Case 2. The numbers a,ba, b, and cc are odd.

If any two of these numbers are equal, say a=ba = b, then acb=a(c1)ac - b = a(c - 1) has a nontrivial odd divisor and cannot be a power of 22. Hence a,ba, b, and cc are distinct. So we may assume without loss of generality that a<b<ca < b < c.

Let α\alpha and β\beta denote the nonnegative integers for which bca=2αbc - a = 2^{\alpha} and acb=2βac - b = 2^{\beta} hold. Clearly, we have α>β\alpha > \beta, and thus 2β2^{\beta} divides
a2αb2β=a(bca)b(acb)=b2a2=(b+a)(ba). a \cdot 2^{\alpha} - b \cdot 2^{\beta} = a(bc - a) - b(ac - b) = b^{2} - a^{2} = (b + a)(b - a).
Since aa is odd, it is not possible that both factors b+ab + a and bab - a are divisible by 44. Consequently, one of them has to be a multiple of 2β12^{\beta - 1}. Hence one of the numbers 2(b+a)2(b + a) and 2(ba)2(b - a) is divisible by 2β2^{\beta} and in either case we have
acb=2β2(a+b). ac - b = 2^{\beta} \leqslant 2(a + b).
This in turn yields (a1)b<acb<4b(a - 1)b < ac - b < 4b and thus a=3a = 3 (recall that aa is odd and larger than 11). Substituting this back into (7) we learn cb+2c \leqslant b + 2. But due to the parity b<cb < c entails that b+2cb + 2 \leqslant c holds as well. So we get c=b+2c = b + 2 and from bca=(b1)(b+3)bc - a = (b - 1)(b + 3) being a power of 22 it follows that b=5b = 5 and c=7c = 7.

Case 3. Among a,ba, b, and cc both parities occur.

Without loss of generality, we suppose that cc is odd and that aba \leqslant b. We are to show that (a,b,c)(a, b, c) is either (2,2,3)(2, 2, 3) or (2,6,11)(2, 6, 11). As at least one of aa and bb is even, the expression abcab - c is odd; since it is also a power of 22, we obtain
abc=1. ab - c = 1.
If a=ba = b, then c=a21c = a^{2} - 1, and from acb=a(a22)ac - b = a(a^{2} - 2) being a power of 22 it follows that both aa and a22a^{2} - 2 are powers of 22, whence a=2a = 2. This gives rise to the solution (2,2,3)(2, 2, 3).

We may suppose a<ba < b from now on. As usual, we let α>β\alpha > \beta denote the integers satisfying
2α=bcaand2β=acb 2^{\alpha} = bc - a \quad \text{and} \quad 2^{\beta} = ac - b
If β=0\beta = 0 it would follow that acb=abc=1ac - b = ab - c = 1 and hence that b=c=1b = c = 1, which is absurd. So β\beta and α\alpha are positive and consequently aa and bb are even. Substituting c=ab1c = ab - 1 into (9) we obtain
2α=ab2(a+b)2β=a2b(a+b) \begin{align*} 2^{\alpha} & = ab^{2} - (a + b) \\ 2^{\beta} & = a^{2}b - (a + b) \end{align*}
The addition of both equation yields 2α+2β=(ab2)(a+b)2^{\alpha} + 2^{\beta} = (ab - 2)(a + b). Now ab2ab - 2 is even but not divisible by 44, so the highest power of 22 dividing a+ba + b is 2β12^{\beta - 1}. For this reason, the equations (10) and (11) show that the highest powers of 22 dividing either of the numbers ab2ab^{2} and a2ba^{2}b is likewise 2β12^{\beta - 1}. Thus there is an integer τ1\tau \geqslant 1 together with odd integers A,BA, B, and CC such that a=2τA,b=2τB,a+b=23τCa = 2^{\tau}A, b = 2^{\tau}B, a + b = 2^{3\tau}C, and β=1+3τ\beta = 1 + 3\tau.

Notice that A+B=22τC4CA + B = 2^{2\tau}C \geqslant 4C. Moreover, (11) entails A2BC=2A^{2}B - C = 2. Thus 8=4A2B4C4A2BABA2(3B1)8 = 4A^{2}B - 4C \geqslant 4A^{2}B - A - B \geqslant A^{2}(3B - 1). Since AA and BB are odd with A<BA < B, this is only possible if A=1A = 1 and B=3B = 3. Finally, one may conclude C=1,τ=1,a=2,b=6C = 1, \tau = 1, a = 2, b = 6, and c=11c = 11. We have thereby found the triple (2,6,11)(2, 6, 11). This completes the discussion of the third case, and hence the solution.

Looking for a route rather than an archive? The track puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.

Source: MathNet, licensed CC-BY-4.0. Statement and solution reproduced as published; topic and difficulty added by this site.