1. Chatbot’s definition on Wikipedia.
https://en.wikipedia.org/wiki/Chatbot
2. MacTear, M., Callejas, Z.: The History of Chatbots.
https://onlim.com/en/the-history-of-chatbots/
3. Goebel, T.: Machine Learning or Linguistic Rules: Two Approaches to Building a Chatbot.
https://www.cmswire.com/digital-experience/machine-learning-or-linguistic-rules-two-approaches-to-building-a-chatbot/
4. Pavel Surmenok, Chatbot Architecture 2016.
https://medium.com/@surmenok/chatbot-architecture-496f5bf820ed
5. Chowdhury, G.G.: Natural language processing. Ann. Rev. Inf. Sci. Technol. 37(1), 51–89 (2003)