He used his feelings of homesickness during this time to write lyrics that compared his current residence, Tokyo, to his hometown of Otaru, Hokkaido.

Critics praised the song's "addictive" electro and minimalist sound, and felt that it was reminiscent of the works of Rei Harakami.

The first of these was the band's single "Boku to Hana", released in May 2012, which was used as the theme song for the Tsuyoshi Kusanagi drama 37-sai de Isha ni Natta Boku: Kenshui Junjō Monogatari,[2] Three months later, the band followed this with the single "Yoru no Odoriko", a song that had been featured in commercials for the design school Mode Gakuen from April 2013.

[15] Soon after the band finished touring, Yamaguchi worked on a jingle for the yobikō Tōshin High School, a song that would grow to become "Sayonara wa Emotion".

[16] While Yamaguchi was working on "Sayonara wa Emotion", he was contacted to write a song for the film Judge!, which he accepted.

[16] Originally the film staff had asked Sakanaction if they could use their 2010 single "Identity" for the theme song, which Yamaguchi thought of as a great honor as a composer.

[16] While he wrote "Eureka", Yamaguchi became sick both physically and mentally, developing hives and becoming disillusioned with the idea of writing music for other entities.

[19] "Eureka" thematically linked to the other leading song on the single, "Good-Bye", which was written about Yamaguchi's decision to continue to make music in Tokyo.

Both editions featured a remix of the song "Eiga" from Sakanaction created by electronic musician Aoki Takamasa.

[28] The video depicts Yamaguchi in a darkened room, behind a row of naked women who are lying down on the ground in a line.

In the final scene, a wider top-down angle shows that the naked women are spelling out the word eureka in capital letters.

Billboard Japan reviewer Takushi Yamaguchi called "Eureka" a "high quality song", feeling that it was an extension of the techno pop material found on the band's previous album, Sakanaction.

[29] CDJournal reviewers felt "Eureka"'s electro and minimalist sound was "addictive" and reminiscent of Rei Harakami.

believed that "Eureka" was proof of Sakanaction's skill at being a ground-breaking electronic musical group, describing the "minimal techno" song as "a miraculous balance of a subtly constructed track and a raw band sound.

[37] All tracks are written by Ichiro YamaguchiPersonnel details were sourced from "Good-Bye" / "Eureka"'s liner notes booklet.