The song played in the end credits of Episode 5 is a reference to Kawaguchi Hiroshi, an actor acclaimed for his leading role in the Chambara film The 47 Ronin who passed away in 1987.
The ending song used in episode six was originally created for a Coca-Cola commercial. The singer, Yu Hayami, would later play Nancy/Rescue Universe in Tomica Hero Rescue Force
The solution for the differential equation in episode 3 is sin(x)/x, at least when n=1.
The solution for the problem they discuss at Sugata's campsite is (Pi * n)/5, where n is any integer.