【Question 4】
In history, the first person to study magic squares as a mathematical problem was Yang Hui, a famous mathematician of the Song Dynasty in China. $\qquad$
【Analysis and Solution】
In a square composed of several neatly arranged numbers, if the sum of the numbers in any row, column, and diagonal is equal, such a chart is called a "magic square." In ancient China, it was known as "He Tu" or "Luo Shu," and also called "transverse and longitudinal diagrams." In the thirteenth century, Chinese Southern Song mathematician Yang Hui was the first in the world to conduct systematic research on magic squares. Fill in " $\sqrt{ }$ ".