This is my major gripe with the article. While useful for intuition, that "..." after the sum is mathematically meaningless. This is also a common issue with qu

This is my major gripe with the article. While useful for intuition, that "..." after the sum is mathematically meaningless. This is also a common issue with quantum mechanics as taught at an introductory level. But it seems the article, similarly to intro QM courses, is more about motivating functional analysis concepts, which is useful as exposition even if not rigorous.