於 (at, in, on, as for)

後書きには『武蔵国狭山丘陵に於いて書く』との記述がある。

There is a mention saying "I have written this in Sayama hills, Musashi Province" in the afterword.

府政に於いては、「憲法を暮らしの中に生かそう」の垂れ幕を京都府庁に掲げた。

Ninagawa's administration hung a banner reading 'Let's Make the Constitution Live in Our Lives' from Kyoto Prefectural Office building.

洋室のフロア等に於いて使用し、座って畳の感触を得ることができる畳風座布団とする。

To provide a floor cushion like a Tatami (rush) mat which can be put on the floor in a western-style room and gives the texture of a Tatami when sitting on it.