From the beginning of the 17th century until 1857, when they were forcefully made to rejoin the outside world, Japan pursued a policy of total isolation (sakoku), allowing no foreigners to come except for one Dutch ship per year under strict control and not allowing any Japanese to leave and return under penalty of death. During this period they developed an extremely interesting indigenous mathematics (wasan). In particular, the two great mathematicians Seki and his student Takebe made many of the same mathematical discoveries as Bernoulli, Euler, Leibniz or others in the West, but often in an almost unrecognizably different form. I will try to give an impression of both the historical context and of the actual mathematics they developed.