Fill in-15-12-9-6-3 0369 ~ ~ by the way, the simpler the method, the better ~ thank you.

Note that this is a arithmetic progression (the difference between every two numbers is the same):

Method 1 (practical n * n, where n is an odd number).

Look at the picture, the steps are as follows:

(1) Fill in the middle at the top 1,

(2) Fill in the upper right grid of 1 with 2 (2 is out of bounds in the upper right grid of 1, so it is the bottom grid of the column), then fill in the upper right grid of 2 with 3, and so on (3 is in the upper right grid of 2, so it is easy to understand why).

(3) If the upper right box has been filled in, then fill in a number in the box below him. The rest are filled in the box at the top right of the number (this is the case with 3 and 6).

Just change the above 1-9 to your number.