$ftextsc{$p$}(x)=x^3-x^2+px+1$

documentclass{article}
begin{document}
$f_p(x) = x^3 - x^2 + px + 1$
end{document}

documentclass{article}
usepackage{amsmath}
begin{document}
$f_{text{prob}}(x) = x^3 - x^2 + px + 1$
end{document}