Concentric magic squares

This work is inspired by MathSoc page which is maintained by Dave Chambers. In the page, a 7x7 magic square which has non-standard properties is shown. The sum of the numbers in each shell square inside the magic square conforms a regular sequence with the same difference 200.
```
        CCCCCCC
        CBBBBBC
        CBAAABC
        CBAcABC
        CBAAABC
        CBBBBBC
        CCCCCCC
```
Therefore the sums of the numbers located at As, Bs, and Cs are 200, 400, and 600 in the magic square. This square is quite interesting. I found the formula which represents the location of each number.
Suppose an integer number i. The location of the number is given by
```
((N*N+(N-1)/2-[i/N]+i%N)%N, (N*N+(N-1)/2-[i/N]-i%N-1)%N)
```
where "x%N" takes a residue of x under the modulus N, and [x] takes the floor integer of x.
Examples would be found at "examples of concentric magic squares".
Mathemusement : Magic Squares and Magic Cubes