Whether 1 should be a prime number or not wasn't clear-cut for centuries. Current consensus has settled on excluding 1 from the definition, but there are examp

Whether 1 should be a prime number or not wasn't clear-cut for centuries. Current consensus has settled on excluding 1 from the definition, but there are examples of publications well into the 20th century that still included 1 as prime. Fascinating read about the subject: https://arxiv.org/pdf/1209.2007.pdf https://arxiv.org/pdf/1209.2007.pdf