Condensing John Randall's exposition:
rot =: (1&|.)on(1&|."1) NB. rotate rows & cols by 1
flip =: |. on |."1 NB. mirror-image about centre
tr =: |: NB. transpose
ref =: tr on flip NB. reflect in counter diagonal
Then
G = rot G --> G = rot^:n G for every n
--> each subdiagonal of G consists of a single repeated number
--> G = ref G
and so
G = ref G {G = rot G}
= tr flip G {defn of ref}
= flip tr G {they are commutative}
= flip G {G = tr G}
Thanks /J
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