documentclass{article}

usepackage{amsmath}
usepackage[latin9]{inputenc}
usepackage{mathptmx}
DeclareMathAlphabet{mathpzc}{OT1}{pzc}{m}{it}

begin{document}

    We define the following equation:
    [mathpzc{S} = sum_{k=0}^ndfrac{1}{k^3}.]


    Now let $mathbf{x}$ be the following vector:
    [mathbf{x}=[x_1, ldots, x_n]^mathsf{T}]

end{document}