
The following problem is investigated: let \(f\) be a polynomial given by the expansion \(f(z)=a_0 p_0(z)+a_1p_1(z)+\cdots+a_np_n(z)\) in terms of orthogonal polynomials. What can be said about the zeros of \(f\) in terms of the zeros of the orthogonal polynomials \(p_j\) and the Fourier coefficients \(a_j\)? The main result is a condition on the (real) Fourier coefficients \(a_j\) which implies that \(f\) has at least \(k\) distinct real zeros of odd multiplicity. If the condition does not hold, then a region is given outside of which \(f\) has at most \((n-k)/2\) pairs of conjugate zeros. As a corollary, a strip is found where the polynomial has at least \(k\) zeros. Another result, of independent interest, is an upper bound for the ratio \(\|f\|/ \|fg\|\), where \(f\) and \(g\) are polynomials of degree \(n\) and \(k\), respectively, and the norms are in \(L^2(\sigma)\), where \(\sigma\) is the orthogonality measure for the polynomials \(p_k\).
Mathematics(all), Numerical Analysis, weighted L2 inequality, zeros of polynomials, Applied Mathematics, Fourier coefficients, orthogonal expansion, Zeros of polynomials, rational functions, and other analytic functions of one complex variable (e.g., zeros of functions with bounded Dirichlet integral), location of zeros, real polynomials, Orthogonal functions and polynomials, general theory of nontrigonometric harmonic analysis, orthogonal polynomials, Analysis, criterion for real zeros
Mathematics(all), Numerical Analysis, weighted L2 inequality, zeros of polynomials, Applied Mathematics, Fourier coefficients, orthogonal expansion, Zeros of polynomials, rational functions, and other analytic functions of one complex variable (e.g., zeros of functions with bounded Dirichlet integral), location of zeros, real polynomials, Orthogonal functions and polynomials, general theory of nontrigonometric harmonic analysis, orthogonal polynomials, Analysis, criterion for real zeros
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