Explanation 20x20 magic square
The 20x20 magic square is an odd multiple of 4. A Franklin panmagic 20x20 square does not exist (a Franklin [not pan]magic square can be constructed with help of the computer).
Methods to construct the 20x20 magic square are:
The composite and concentric 20x20 magic square are 'specials'.
Symmetric transformation and basic key method (ultra magic) lead to ultra magic 20x20 squares.
The remaining methods lead to most perfect magic 20x20 squares.