Hansen's Theorem is a result in the field of mathematics, specifically in the area of functional analysis. It provides conditions under which a certain type of operator, known as a bounded linear operator, can be approximated by simpler operators. This theorem is particularly useful in understanding the behavior of operators in infinite-dimensional spaces. The theorem is named after J. L. Hansen, who contributed to the study of operator theory. By establishing a framework for approximation, Hansen's Theorem helps mathematicians analyze complex systems and solve problems related to Hilbert spaces and Banach spaces.