Do there exist polynomials such that holds identically?
Solution
No, there do not. solution:} Suppose the contrary. By setting in succession, we see that the polynomials are linear combinations of and . 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 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. solution:} It is equivalent (by relabeling and rescaling) to show that cannot be written as . Write , , , . We now start comparing coefficients of . By comparing coefficients of and , we get The first equation says that and cannot both vanish, and and cannot both vanish. The second equation says that when , where both sides should be viewed in (and neither is undetermined if ). But then contradicting the equation . solution:} We work over the complex numbers, in which we have a primitive cube root 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 , the rational function must vanish identically (that is, coefficient by coefficient). If one of the polynomials, say , vanished identically, then one of or would also, and the desired inequality could not hold. So none of them vanish identically, and we can write Likewise, Put ; then we have identically. That is, . Since and have no common factor (otherwise would have a factor divisible only by , which it doesn't since it doesn't vanish identically for any particular ), divides . Since they have the same degree, they are equal up to scalars. It follows that one of is a polynomial in alone, and likewise for (with the same power of ). If and , or and , are polynomials in , then and are divisible by , but we know and have no common factor. Hence and are polynomials in . Likewise, and are polynomials in . But then is a polynomial in and , contradiction. 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 by another value if working in characteristic 2.)