documentclass{article}

usepackage[intlimits]{kpfonts}
usepackage{amsmath,mathtools}

begin{document}
begin{equation}
I = int_{t_{k}}^{t_{k+1}} f(t) dt
end{equation}

begin{equation}
SMOP = smashoperator{int_{t_{k}}^{t_{k+1}}} f(t) dt
end{equation}
end{document}