- 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".