[1] 位取り記数法 - Wikipedia (2018-12-08 11:22:22 +09:00) https://ja.wikipedia.org/wiki/%E4%BD%8D%E5%8F%96%E3%82%8A%E8%A8%98%E6%95%B0%E6%B3%95#%E5%9F%BA%E6%95%B0%E3%81%AE%E6%98%8E%E8%A8%98

[2] Quaternary numeral system - Wikipedia (2018-12-07 15:51:15 +09:00) https://en.wikipedia.org/wiki/Quaternary_numeral_system