AlgebraDifficulty 8.1ShortlistProve itUnited States
Suppose a, b, and c are three complex numbers with product 1. Assume that none of a, b, and c are real or have absolute value 1. Define p=(a+b+c)+(a1+b1+c1)andq=ba+cb+ac. Given that both p and q are real numbers, find all possible values of the ordered pair (p,q).
Solution
Let us denote a=xy, b=yz, c=zx, where x,y,z are nonzero complex numbers. Then p+3=3+cyc∑(yx+xy)=3+xyzx2(y+z)+y2(z+x)+z2(x+y)=xyz(x+y+z)(xy+yz+zx). q−3=−3+cyc∑zxy2=xyzx3+y3+z3−3xyz=xyz(x+y+z)(x2+y2+z2−xy−yz−zx).
Scale x,y,z in such a way that x+y+z is nonzero and real; hence so is xyz. Thus, as p+3∈R, we conclude xy+yz+zx∈R as well. Hence, x,y,z are the roots of a cubic with real coefficients. Thus, * either all three of {x,y,z} are real (which implies a,b,c∈R), * or two of {x,y,z} are a complex conjugate pair (which implies one of a,b,c has absolute value 1). Both of these were forbidden by hypothesis.
Therefore, the only possibility is x+y+z=0, which gives p=−3, q=3.
Second solution, found by contestants
The main idea is to make the substitution x=a+c1,y=b+a1,z=c+b1. Then we can check that x+y+zxy+yz+zxxyz=p=p+q+3=p+2. Therefore x,y,z are the roots of a cubic with real coefficients. As in the previous solution, we note that this cubic must either have all real roots, or a complex conjugate pair of roots. We also have the relation a(y+1)=ab+a+1=x+1, and likewise b(z+1)=y+1, c(x+1)=z+1. This means that if any of x,y,z are equal to −1, then all are equal to −1. Assume for the sake of contradiction that none are equal to −1. In the case where the cubic has three real roots, a=y+1x+1 would be real. On the other hand, if there is a complex conjugate pair (without loss of generality, x and y) then a has magnitude 1. Therefore this cannot occur.
We conclude that x=y=z=−1, so p=−3 and q=3. The solutions (a,b,c) can then be parameterized as (a,−1−a1,−1+a1). To construct a solution, we need to choose a specific value of a such that none of the wrong conditions hold; when a=2i, say, we obtain the solution (2i,−1+2i,5−1+2i).
Third solution by Luke Robitaille and Daniel Zhu
The answer is p=−3 and q=3. Let's first prove that no other (p,q) work.
Let e1=a+b+c and e2=a−1+b−1+c−1=ab+ac+bc. Also, let f=e1e2. Note that p=e1+e2.
Our main insight is to consider the quantity q′=ab+bc+ca. Note that f=q+q′+3. Also, qq′=3+bca2+acb2+abc2+a2bc+b2ac+c2ab=3+a3+b3+c3+a−3+b−3+c−3=9+a3+b3+c3−3abc+a−3+b−3+c−3−3a−1b−1c−1=9+e1(e12−3e2)+e2(e22−3e1)=9+e13+e23−6e1e2=9+p(p2−3f)−6f=p3−(3p+6)f+9.
As a result, the quadratic with roots q and q′ is x2−(f−3)x+(p3−(3p+6)f+9), which implies that q2−qf+3q+p3−(3p+6)f+9=0⟺(3p+q+6)f=p3+q2+3q+9.
Claim — f is not real.
Proof. Suppose f is real. Since (x−e1)(x−e2)=x2−px+f, there are two cases: * e1 and e2 are real. Then, a,b, and c are the roots of x3−e1x2+e2x−1, and since every cubic with real coefficients has at least one real root, at least one of a,b, and c is real, contradiction. * e1 and e2 are conjugates. Then, the polynomial x3−eˉ2x2+eˉ1x−1, which has roots aˉ−1,bˉ−1, and cˉ−1, is the same as the polynomial with a,b,c as roots. We conclude that the multiset {a,b,c} is invariant under inversion about the unit circle, so one of a,b, and c must lie on the unit circle. This is yet another contradiction. □
As a result, we know that 3p+q+6=p3+q2+3q+9=0. The second miracle is that substituting q=−3p−6 into q2+3q+p3+9=0, we get 0=p3+9p2+27p+27=(p+3)3, so p=−3. Thus q=3.
It remains to construct valid a,b, and c. To do this, let's pick some e1, let e2=−3−e1, and let a,b, and c be the roots of x3−e1x2+e2x−1. It is clear that this guarantees p=−3. By our above calculations, q and q′ are the roots of the quadratic x2−(f−3)x+(3f−18), so one of q and q′ must be 3; by changing the order of a,b, and c if needed, we can guarantee this to be q. It suffices to show that for some choice of e1, none of a,b, or c are real or lie on the unit circle. To do this, note that we can rewrite x3−e1x2+(−3−e1)x−1=0 as e1=x2+xx3−3x−1, so all we need is a value of e1 that is not x2+xx3−3x−1 for any real x or x on the unit circle. One way to do this is to choose any nonreal e1 with ∣e1∣<1/2. This clearly rules out any real x. Also, if ∣x∣=1, by the triangle inequality ∣x3−3x−1∣≥∣3x∣−∣x3∣−∣1∣=1 and ∣x2+x∣≤2, so x2+xx3−3x−1≥1/2.
Therefore, the only possible values of (p,q) are (−3,3).
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