Determine all triples $(a, b, c)$ of positive integers for which $a b-c, b c-a$, and $c a-b$ are powers of 2. Explanation: A power of 2 is an integer of the form $2^{n}$, where $n$ denotes some nonnegative integer. (Serbia) Answer. There are sixteen such triples, namely $(2,2,2)$, the three permutations of $(2,2,3)$, and the six permutations of each of $(2,6,11)$ and $(3,5,7)$.
It can easily be verified that these sixteen triples are as required. Now let ( $a, b, c$ ) be any triple with the desired property. If we would have $a=1$, then both $b-c$ and $c-b$ were powers of 2, which is impossible since their sum is zero; because of symmetry, this argument shows $a, b, c \geqslant 2$.  Case 1. Among $a, b$, and $c$ there are at least two equal numbers. Without loss of generality we may suppose that $a=b$. Then $a^{2}-c$ and $a(c-1)$ are powers of 2. The latter tells us that actually $a$ and $c-1$ are powers of 2. So there are nonnegative integers $\alpha$ and $\gamma$ with $a=2^{\alpha}$ and $c=2^{\gamma}+1$. Since $a^{2}-c=2^{2 \alpha}-2^{\gamma}-1$ is a power of 2 and thus incongruent to -1 modulo 4, we must have $\gamma \leqslant 1$. Moreover, each of the terms $2^{2 \alpha}-2$ and $2^{2 \alpha}-3$ can only be a power of 2 if $\alpha=1$. It follows that the triple $(a, b, c)$ is either $(2,2,2)$ or $(2,2,3)$.  Case 2. The numbers $a, b$, and $c$ are distinct. Due to symmetry we may suppose that  $$ 2 \leqslant a\beta>\gamma . $$  Depending on how large $a$ is, we divide the argument into two further cases. Case 2.1. $\quad a=2$. We first prove that $\gamma=0$. Assume for the sake of contradiction that $\gamma>0$. Then $c$ is even by (4) and, similarly, $b$ is even by (5) and (3). So the left-hand side of (2) is congruent to 2 modulo 4, which is only possible if $b c=4$. As this contradicts (1), we have thereby shown that $\gamma=0$, i.e., that $c=2 b-1$.  Now (3) yields $3 b-2=2^{\beta}$. Due to $b>2$ this is only possible if $\beta \geqslant 4$. If $\beta=4$, then we get $b=6$ and $c=2 \cdot 6-1=11$, which is a solution. It remains to deal with the case $\beta \geqslant 5$. Now (2) implies  $$ 9 \cdot 2^{\alpha}=9 b(2 b-1)-18=(3 b-2)(6 b+1)-16=2^{\beta}\left(2^{\beta+1}+5\right)-16 $$  and by $\beta \geqslant 5$ the right-hand side is not divisible by 32. Thus $\alpha \leqslant 4$ and we get a contradiction to (5).  Case 2.2. $\quad a \geqslant 3$. Pick an integer $\vartheta \in\{-1,+1\}$ such that $c-\vartheta$ is not divisible by 4. Now  $$ 2^{\alpha}+\vartheta \cdot 2^{\beta}=\left(b c-a \vartheta^{2}\right)+\vartheta(c a-b)=(b+a \vartheta)(c-\vartheta) $$  is divisible by $2^{\beta}$ and, consequently, $b+a \vartheta$ is divisible by $2^{\beta-1}$. On the other hand, $2^{\beta}=a c-b>$ $(a-1) c \geqslant 2 c$ implies in view of (1) that $a$ and $b$ are smaller than $2^{\beta-1}$. All this is only possible if $\vartheta=1$ and $a+b=2^{\beta-1}$. Now (3) yields  $$ a c-b=2(a+b) $$  whence $4 b>a+3 b=a(c-1) \geqslant a b$, which in turn yields $a=3$. So (6) simplifies to $c=b+2$ and (2) tells us that $b(b+2)-3=(b-1)(b+3)$ is a power of 2. Consequently, the factors $b-1$ and $b+3$ are powers of 2 themselves. Since their difference is 4, this is only possible if $b=5$ and thus $c=7$. Thereby the solution is complete.