The startup, Anthropic, has become one of OpenAI’s most formidable competitors. : Amazon.com Inc. says it’s investing an additional $2.75 billion in Anthropic, completing a deal it made last year to back the artificial intelligence startup and expand a partnership between the companies. The infusion brings Amazon’s total investment in Anthropic, a well-regarded builder of AI tools able to generate text and analysis, to $4 billion, following an earlier investment announced in September.