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Algebra Difficulty 8.1 Shortlist Prove it United States

Suppose aa, bb, and cc are three complex numbers with product 11. Assume that none of aa, bb, and cc are real or have absolute value 11. Define
p=(a+b+c)+(1a+1b+1c)andq=ab+bc+ca. p = (a + b + c) + \left( \frac{1}{a} + \frac{1}{b} + \frac{1}{c} \right) \quad \text{and} \quad q = \frac{a}{b} + \frac{b}{c} + \frac{c}{a}.
Given that both pp and qq are real numbers, find all possible values of the ordered pair (p,q)(p, q).

Solution

Let us denote a=yxa = \frac{y}{x}, b=zyb = \frac{z}{y}, c=xzc = \frac{x}{z}, where x,y,zx, y, z are nonzero complex numbers. Then
p+3=3+cyc(xy+yx)=3+x2(y+z)+y2(z+x)+z2(x+y)xyz=(x+y+z)(xy+yz+zx)xyz. p+3 = 3 + \sum_{\text{cyc}} \left(\frac{x}{y} + \frac{y}{x}\right) = 3 + \frac{x^2(y+z) + y^2(z+x) + z^2(x+y)}{xyz} \\ = \frac{(x+y+z)(xy+yz+zx)}{xyz}.
q3=3+cycy2zx=x3+y3+z33xyzxyz=(x+y+z)(x2+y2+z2xyyzzx)xyz. q-3 = -3 + \sum_{\text{cyc}} \frac{y^2}{zx} = \frac{x^3 + y^3 + z^3 - 3xyz}{xyz} \\ = \frac{(x+y+z)(x^2+y^2+z^2-xy-yz-zx)}{xyz}.

R3(p+3)+(q3)=(x+y+z)(x2+y2+z2+2(xy+yz+zx))xyz=(x+y+z)3xyz. \mathbb{R} \ni 3(p+3) + (q-3) = \frac{(x+y+z)(x^2+y^2+z^2+2(xy+yz+zx))}{xyz} = \frac{(x+y+z)^3}{xyz}.

Scale x,y,zx, y, z in such a way that x+y+zx + y + z is nonzero and real; hence so is xyzxyz. Thus, as p+3Rp + 3 \in \mathbb{R}, we conclude xy+yz+zxRxy + yz + zx \in \mathbb{R} as well. Hence, x,y,zx, y, z are the roots of a cubic with real coefficients. Thus,
* either all three of {x,y,z}\{x, y, z\} are real (which implies a,b,cRa, b, c \in \mathbb{R}),
* or two of {x,y,z}\{x, y, z\} are a complex conjugate pair (which implies one of a,b,ca, b, c has absolute value 11).
Both of these were forbidden by hypothesis.

Therefore, the only possibility is x+y+z=0x + y + z = 0, which gives p=3p = -3, q=3q = 3.

Second solution, found by contestants

The main idea is to make the substitution
x=a+1c,y=b+1a,z=c+1b. x = a + \frac{1}{c}, \quad y = b + \frac{1}{a}, \quad z = c + \frac{1}{b}.
Then we can check that
x+y+z=pxy+yz+zx=p+q+3xyz=p+2. \begin{aligned} x+y+z &= p \\ xy+yz+zx &= p+q+3 \\ xyz &= p+2. \end{aligned}
Therefore x,y,zx, 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+1a(y+1) = ab + a + 1 = x + 1, and likewise b(z+1)=y+1b(z+1) = y + 1, c(x+1)=z+1c(x+1) = z + 1. This means that if any of x,y,zx, y, z are equal to 1-1, then all are equal to 1-1.
Assume for the sake of contradiction that none are equal to 1-1. In the case where the cubic has three real roots, a=x+1y+1a = \frac{x+1}{y+1} would be real. On the other hand, if there is a complex conjugate pair (without loss of generality, xx and yy) then aa has magnitude 11. Therefore this cannot occur.

We conclude that x=y=z=1x = y = z = -1, so p=3p = -3 and q=3q = 3. The solutions (a,b,c)(a, b, c) can then be parameterized as (a,11a,11+a)(a, -1 - \frac{1}{a}, -\frac{1}{1+a}). To construct a solution, we need to choose a specific value of aa such that none of the wrong conditions hold; when a=2ia = 2i, say, we obtain the solution (2i,1+i2,1+2i5)(2i, -1 + \frac{i}{2}, \frac{-1+2i}{5}).

Third solution by Luke Robitaille and Daniel Zhu

The answer is p=3p = -3 and q=3q = 3. Let's first prove that no other (p,q)(p, q) work.

Let e1=a+b+ce_1 = a + b + c and e2=a1+b1+c1=ab+ac+bce_2 = a^{-1} + b^{-1} + c^{-1} = ab + ac + bc. Also, let f=e1e2f = e_1 e_2. Note that p=e1+e2p = e_1 + e_2.

Our main insight is to consider the quantity q=ba+cb+acq' = \frac{b}{a} + \frac{c}{b} + \frac{a}{c}. Note that f=q+q+3f = q + q' + 3. Also,
qq=3+a2bc+b2ac+c2ab+bca2+acb2+abc2=3+a3+b3+c3+a3+b3+c3=9+a3+b3+c33abc+a3+b3+c33a1b1c1=9+e1(e123e2)+e2(e223e1)=9+e13+e236e1e2=9+p(p23f)6f=p3(3p+6)f+9. \begin{aligned} qq' &= 3 + \frac{a^2}{bc} + \frac{b^2}{ac} + \frac{c^2}{ab} + \frac{bc}{a^2} + \frac{ac}{b^2} + \frac{ab}{c^2} \\ &= 3 + a^3 + b^3 + c^3 + a^{-3} + b^{-3} + c^{-3} \\ &= 9 + a^3 + b^3 + c^3 - 3abc + a^{-3} + b^{-3} + c^{-3} - 3a^{-1}b^{-1}c^{-1} \\ &= 9 + e_1(e_1^2 - 3e_2) + e_2(e_2^2 - 3e_1) \\ &= 9 + e_1^3 + e_2^3 - 6e_1e_2 \\ &= 9 + p(p^2 - 3f) - 6f \\ &= p^3 - (3p + 6)f + 9. \end{aligned}

As a result, the quadratic with roots qq and qq' is x2(f3)x+(p3(3p+6)f+9)x^2 - (f-3)x + (p^3 - (3p+6)f + 9), which implies that
q2qf+3q+p3(3p+6)f+9=0    (3p+q+6)f=p3+q2+3q+9. q^2 - qf + 3q + p^3 - (3p+6)f + 9 = 0 \iff (3p+q+6)f = p^3 + q^2 + 3q + 9.

Claimff is not real.

Proof. Suppose ff is real. Since (xe1)(xe2)=x2px+f(x - e_1)(x - e_2) = x^2 - px + f, there are two cases:
* e1e_1 and e2e_2 are real. Then, a,ba, b, and cc are the roots of x3e1x2+e2x1x^3 - e_1x^2 + e_2x - 1, and since every cubic with real coefficients has at least one real root, at least one of a,ba, b, and cc is real, contradiction.
* e1e_1 and e2e_2 are conjugates. Then, the polynomial x3eˉ2x2+eˉ1x1x^3 - \bar{e}_2x^2 + \bar{e}_1x - 1, which has roots aˉ1,bˉ1\bar{a}^{-1}, \bar{b}^{-1}, and cˉ1\bar{c}^{-1}, is the same as the polynomial with a,b,ca, b, c as roots. We conclude that the multiset {a,b,c}\{a, b, c\} is invariant under inversion about the unit circle, so one of a,ba, b, and cc must lie on the unit circle. This is yet another contradiction. \square

As a result, we know that 3p+q+6=p3+q2+3q+9=03p + q + 6 = p^3 + q^2 + 3q + 9 = 0. The second miracle is that substituting q=3p6q = -3p - 6 into q2+3q+p3+9=0q^2 + 3q + p^3 + 9 = 0, we get
0=p3+9p2+27p+27=(p+3)3, 0 = p^3 + 9p^2 + 27p + 27 = (p + 3)^3,
so p=3p = -3. Thus q=3q = 3.

It remains to construct valid a,ba, b, and cc. To do this, let's pick some e1e_1, let e2=3e1e_2 = -3 - e_1, and let a,ba, b, and cc be the roots of x3e1x2+e2x1x^3 - e_1x^2 + e_2x - 1. It is clear that this guarantees p=3p = -3. By our above calculations, qq and qq' are the roots of the quadratic x2(f3)x+(3f18)x^2 - (f-3)x + (3f-18), so one of qq and qq' must be 33; by changing the order of a,ba, b, and cc if needed, we can guarantee this to be qq. It suffices to show that for some choice of e1e_1, none of a,ba, b, or cc are real or lie on the unit circle.
To do this, note that we can rewrite x3e1x2+(3e1)x1=0x^3 - e_1x^2 + (-3 - e_1)x - 1 = 0 as
e1=x33x1x2+x, e_1 = \frac{x^3 - 3x - 1}{x^2 + x},
so all we need is a value of e1e_1 that is not x33x1x2+x\frac{x^3-3x-1}{x^2+x} for any real xx or xx on the unit circle. One way to do this is to choose any nonreal e1e_1 with e1<1/2|e_1| < 1/2. This clearly rules out any real xx. Also, if x=1|x| = 1, by the triangle inequality x33x13xx31=1|x^3 - 3x - 1| \ge |3x| - |x^3| - |1| = 1 and x2+x2|x^2 + x| \le 2, so x33x1x2+x1/2\left|\frac{x^3-3x-1}{x^2+x}\right| \ge 1/2.

Therefore, the only possible values of (p,q)(p, q) are (3,3)(-3, 3).

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