documentclass{article}
usepackage{amssymb,xcolor,stackengine}
stackMath
defopendot{mathop{stackinset{r}{}{b}{}{color{white}rule{.55ex}{1.5ex}}{odot}}}
begin{document}
$sum_{n=0}^{+infty} a_n opendot sum_{n=0}^{+infty} b_n :=
  sum_{n=0}^{+infty} c_n$
end{document}