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Number theory Difficulty 7.0 National olympiad, round 2 Prove it Argentina

Find all triplets {a,b,c}\{a, b, c\} of coprime positive integers (not necessarily pairwise coprime) such that a+b+ca+b+c divides simultaneously the three numbers a12+b12+c12a^{12}+b^{12}+c^{12}, a23+b23+c23a^{23}+b^{23}+c^{23}, and a11004+b11004+c11004a^{11004}+b^{11004}+c^{11004}.

Solution

Assume {a,b,c}\{a, b, c\} is a triple satisfying the required conditions. For every positive integer rr, let Sr=ar+br+crS_r = a^r + b^r + c^r. We denote S=S1S = S_1.

First, let us show that SS divides S11k+1S_{11k+1} for every non-negative integer kk. We proceed by induction on kk. For k=0,1,2k = 0, 1, 2, the claim is true by our assumptions on {a,b,c}\{a, b, c\}. Let k2k \ge 2 and assume the result holds for k2,k1k-2, k-1 and kk; we will see it is true for k+1k+1. We have that
S11k+1S11=(a11k+1+b11k+1+c11k+1)(a11+b11+c11)=a11(k+1)+1+b11(k+1)+1+c11(k+1)+1+(ab)11(a11(k1)+1+b11(k1)+1)+(ac)11(a11(k1)+1+c11(k1)+1)+(bc)11(b11(k1)+1+c11(k1)+1)=S11(k+1)+1+((ab)11+(ac)11+(bc)11)S11(k1)+1(abc)11(a11(k2)+1+b11(k2)+1+c11(k2)+1)=S11(k+1)+1+((ab)11+(ac)11+(bc)11)S11(k1)+1(abc)11S11(k2)+1. \begin{aligned} S_{11k+1} \cdot S_{11} &= (a^{11k+1} + b^{11k+1} + c^{11k+1})(a^{11} + b^{11} + c^{11}) \\ &= a^{11(k+1)+1} + b^{11(k+1)+1} + c^{11(k+1)+1} + (ab)^{11}(a^{11(k-1)+1} + b^{11(k-1)+1}) \\ &\quad + (ac)^{11}(a^{11(k-1)+1} + c^{11(k-1)+1}) + (bc)^{11}(b^{11(k-1)+1} + c^{11(k-1)+1}) \\ &= S_{11(k+1)+1} + ((ab)^{11} + (ac)^{11} + (bc)^{11}) S_{11(k-1)+1} - \\ &\quad (abc)^{11}(a^{11(k-2)+1} + b^{11(k-2)+1} + c^{11(k-2)+1}) \\ &= S_{11(k+1)+1} + ((ab)^{11} + (ac)^{11} + (bc)^{11}) S_{11(k-1)+1} - (abc)^{11} S_{11(k-2)+1}. \end{aligned}
By the induction assumption, SS divides S11k+1S_{11k+1}, S11(k1)+1S_{11(k-1)+1} and S11(k2)+1S_{11(k-2)+1}; therefore, the above equality implies that SS divides S11(k+1)+1S_{11(k+1)+1}, as we wanted to prove.

Consider the factorization x3+y3+z33xyz=(x+y+z)(x2+y2+z2xyxzyz)x^3+y^3+z^3-3xyz = (x+y+z)(x^2+y^2+z^2-xy-xz-yz). If x,y,zx, y, z are integers, it implies that every divisor of x+y+zx+y+z also divides x3+y3+z33xyzx^3+y^3+z^3-3xyz. By taking x=a11004x = a^{11004}, y=b11004y = b^{11004} and z=c11004z = c^{11004}, and recalling that SS divides S11004S_{11004}, we deduce that SS divides S330123(abc)11004S_{33012} - 3(abc)^{11004}. Since 330121(mod11)33012 \equiv 1 \pmod{11}, we know that SS divides S33012S_{33012}; therefore, SS divides 3(abc)110043(abc)^{11004}.

Assume p>3p > 3 is a prime factor of SS. Since pp divides 3(abc)110043(abc)^{11004}, then, it divides a,ba, b or cc. With no loss of generality, assume pp divides aa; then, pp divides b+cb+c, as it divides S=a+b+cS = a+b+c. But we also have that pp divides a12+b12+c12a^{12} + b^{12} + c^{12}, and looking modulo pp, we get that a12+b12+c12012+b12+(b)122b12(modp)a^{12} + b^{12} + c^{12} \equiv 0^{12} + b^{12} + (-b)^{12} \equiv 2b^{12} \pmod{p}. It follows that pp divides bb and, as a consequence, it divides cc, contradicting the fact that a,ba, b and cc are coprime. We conclude that SS does not have a prime divisor greater than 3.

Hence, S=2x3yS = 2^x3^y for non-negative integers xx and yy. Finally, we will show that x,y1x, y \le 1. If x2x \ge 2, we have that a12+b12+c120(mod4)a^{12}+b^{12}+c^{12} \equiv 0 \pmod 4. As the quadratic residues modulo 4 are 0 and 1, the only possibility is that abc0(mod2)a \equiv b \equiv c \equiv 0 \pmod 2, contradicting the coprimality of a,b,ca, b, c. Similarly, if y2y \ge 2, we have that a12+b12+c120(mod9)a^{12} + b^{12} + c^{12} \equiv 0 \pmod 9 but, taking into account that for an integer mm, the possible residues of m6m^6 modulo 9 are 0 and 1, this implies that abc0(mod3)a \equiv b \equiv c \equiv 0 \pmod 3, which is again a contradiction.

Therefore, the possible values of S=a+b+cS = a+b+c are 3 and 6, since a,b,ca, b, c are positive integers and, consequently, the possible triples {a,b,c}\{a, b, c\} are {1,1,1}\{1, 1, 1\}, {1,2,3}\{1, 2, 3\}, {1,1,4}\{1, 1, 4\} and {2,2,2}\{2, 2, 2\}. It is clear that the first one satisfies the conditions and that the last one is not a solution because a,b,ca, b, c are not coprime. Now, {1,2,3}\{1, 2, 3\} is not a solution either, since 1+2+3=61+2+3=6 does not divide 112+212+3121^{12} + 2^{12} + 3^{12} (this number has residue 2 modulo 3). To check that {1,1,4}\{1, 1, 4\} satisfies the conditions, it suffices to note that 1r+1r+4r0(mod2)1^r + 1^r + 4^r \equiv 0 \pmod 2 and 1r+1r+4r0(mod3)1^r + 1^r + 4^r \equiv 0 \pmod 3 for every positive integer rr.

We conclude that the solutions are {1,1,1}\{1, 1, 1\} and {1,1,4}\{1, 1, 4\}.

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