This was to be a topic to which he made many important contributions, and we refer again to this work below. His 1908 paper was on the wave equation.
Ito ay upang maging isang topic na kung saan siya ginawa ng maraming mahalagang mga kontribusyon, at muli naming sumangguni sa trabaho na ito sa ibaba. 1908 ay ang kanyang papel sa kumaway equation.