Being Icelandic and speaking the language (which is the closest language to Old Norse), I noticed that Norse mythology is prevalent throughout this series. For example, immediately on episode 18 during Operation Valkyria, Okabe is on the date with Ruka at the cafe "Völuspá Café". ThisThe sign can be seen when they enter the cafe and again when they exit on episode 18.
Völuspá is the first and best known poem of the Edda (collection of Old Norse poems). In the 19th verse of Völuspá, the 3 Norns - Urður, Verðandi, and Skuld are talked about being home at 1one of the 3three roots attached to Yggdrasill, the tree of life. In the 30th verse, Valkyries are gathering because a warrior has fallen and they are preparing to bring him to the afterlife, Valhöll.
Considering all the Norse mythology in this series and that the sign is shown so clearly, it is worth mentioning.