Hadwiger Nelson problem is the unsolved problem in mathematics, about minimum chromatic number of unit distance graph. en.wikipedia.org For planar graph, four color theorem is famous, but the unit distance graph allows intersection of edge…

scrapbox.io I found interesting problem related to separation of variables. We can see separation of variables not only in ODE problem, but PDE problem. www.sciencedirect.com For PDE, it is convenient because we can decompose into time dep…