$\newcommand{\CapPsi}{\mathop{\vphantom{\sum}\mathchoice{\vcenter{\huge\Psi}}{\vcenter{\Large \Psi}}{\Psi}{\Psi}}}$
$$\sum\CapPsi_{b=1}^a\sum^b_{n_b=0}k_n=\sum^1_{n_1=0}\sum^2_{n_2=0}\dots\sum^a_{n_a=0}k_n$$
$$\prod\CapPsi_{b=1}^a\prod^b_{n_b=0}k_n=\prod^1_{n_1=0}\prod^2_{n_2=0}\dots\prod^a_{n_a=0}k_n$$