関山仙太夫 (Japanese Sekiyama Sendayū) 1784-1859. Samurai, amateur player of 5 dan professional strength. Famous for his 20-game match against Honinbo Shusaku.
valerio: correct name of this player is Sekiyama Sendayu, I think.
Bob McGuigan: These are just alternative romanizations of his Japanese name.
bugcat: imo Sendayū properly represents せんだゆう, and Sendaiū represents せんだいうう. I'd expect the former spelling to be correct.
John F. Although you can occasionally find Japanese sources that give this rather common name ending as -aiu in kana, the norm in Japan now is to write it as -da-yuu. (See e.g. the Nihon Seimei Yomifuri Jiten), which in normal English editorial usage becomes -dayu.