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Algebra Difficulty 8.4 Shortlist Find the answer

Do there exist polynomials a(x),b(x),c(y),d(y)a(x), b(x), c(y), d(y) such that 1+xy+x2y2=a(x)c(y)+b(x)d(y)1 + x y + x^2 y^2 = a(x) c(y) + b(x) d(y) holds identically?

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

No, there do not. First\textbf{First} solution:} Suppose the contrary. By setting y=1,0,1y=-1,0,1 in succession, we see that the polynomials 1x+x2,1,1+x+x21-x+x^2, 1, 1+x+x^2 are linear combinations of a(x)a(x) and b(x)b(x). But these three polynomials are linearly independent, so cannot all be written as linear combinations of two other polynomials, contradiction. Alternate formulation: the given equation expresses a diagonal matrix with 1,1,11,1,1 and zeroes on the diagonal, which has rank 3, as the sum of two matrices of rank 1. But the rank of a sum of matrices is at most the sum of the ranks of the individual matrices. Second\textbf{Second} solution:} It is equivalent (by relabeling and rescaling) to show that 1+xy+x2y21 + xy + x^2y^2 cannot be written as a(x)d(y)b(x)c(y)a(x) d(y) - b(x) c(y). Write a(x)=aixia(x) = \sum a_i x^i, b(x)=bixib(x) = \sum b_i x^i, c(y)=cjyjc(y) = \sum c_j y^j, d(y)=djyjd(y) = \sum d_j y^j. We now start comparing coefficients of 1+xy+x2y21 + xy + x^2 y^2. By comparing coefficients of 1+xy+x2y21+xy + x^2y^2 and a(x)d(y)b(x)c(y)a(x)d(y) - b(x)c(y), we get 1=aidibici(i=0,1,2)0=aidjbicj(ij).\begin{align*} 1 &= a_id_i - b_i c_i \qquad (i=0,1,2)\\ 0 &= a_id_j - b_i c_j \qquad (i \neq j). \end{align*} The first equation says that aia_i and bib_i cannot both vanish, and cic_i and did_i cannot both vanish. The second equation says that ai/bi=cj/dja_i/b_i = c_j/d_j when iji \neq j, where both sides should be viewed in \text{} (and neither is undetermined if i,j{0,1,2}i,j \in \{0,1,2\}). But then a0/b0=c1/d1=a2/b2=c0/d0 a_0/b_0 = c_1/d_1 = a_2/b_2 = c_0/d_0 contradicting the equation a0d0b0c0=1a_0d_0 - b_0c_0 = 1. Third\textbf{Third} solution:} We work over the complex numbers, in which we have a primitive cube root ω\omega of 1. We also use without further comment unique factorization for polynomials in two variables over a field. And we keep the relabeling of the second solution. Suppose the contrary. Since 1+xy+x2y2=(1xy/ω)(1xy/ω2)1+xy+x^2y^2 = (1 - xy/\omega)(1 - xy/\omega^2), the rational function a(ω/y)d(y)b(ω/y)c(y)a(\omega/y) d(y) - b(\omega/y) c(y) must vanish identically (that is, coefficient by coefficient). If one of the polynomials, say aa, vanished identically, then one of bb or cc would also, and the desired inequality could not hold. So none of them vanish identically, and we can write c(y)d(y)=a(ω/y)b(ω/y). \frac{c(y)}{d(y)} = \frac{a(\omega/y)}{b(\omega/y)}. Likewise, c(y)d(y)=a(ω2/y)b(ω2/y). \frac{c(y)}{d(y)}= \frac{a(\omega^2/y)}{b(\omega^2/y)}. Put f(x)=a(x)/b(x)f(x) = a(x)/b(x); then we have f(ωx)=f(x)f(\omega x) = f(x) identically. That is, a(x)b(ωx)=b(x)a(ωx)a(x) b(\omega x) = b(x) a(\omega x). Since aa and bb have no common factor (otherwise 1+xy+x2y21+xy+x^2y^2 would have a factor divisible only by xx, which it doesn't since it doesn't vanish identically for any particular xx), a(x)a(x) divides a(ωx)a(\omega x). Since they have the same degree, they are equal up to scalars. It follows that one of a(x),xa(x),x2a(x)a(x), xa(x), x^2a(x) is a polynomial in x3x^3 alone, and likewise for bb (with the same power of xx). If xa(x)xa(x) and xb(x)xb(x), or x2a(x)x^2 a(x) and x2b(x)x^2 b(x), are polynomials in x3x^3, then aa and bb are divisible by xx, but we know aa and bb have no common factor. Hence a(x)a(x) and b(x)b(x) are polynomials in x3x^3. Likewise, c(y)c(y) and d(y)d(y) are polynomials in y3y^3. But then 1+xy+x2y2=a(x)d(y)b(x)c(y)1 + xy + x^2 y^2 = a(x)d(y) - b(x) c(y) is a polynomial in x3x^3 and y3y^3, contradiction. Note:\textbf{Note:} The third solution only works over fields of characteristic not equal to 3, whereas the other two work over arbitrary fields. (In the first solution, one must replace 1-1 by another value if working in characteristic 2.)

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