Friday, 9 March 2012

From the Magic Square to the Magic Torus

This post is a translation of the article "Passage du Carré Magique au Tore Magique," first published in French on the 11th November 2011.

To visualise a magic torus let's begin with the Frénicle index square n° 91 (Bernard Frénicle de Bessy's classification of 880 fourth order magic squares - published posthumously in 1693 in his book "Des Quarrez Magiques" - see the listing on Harvey Heinz's site - Frénicle n°s 1 - 200, Frénicle n°s 201 - 400, Frénicle n°s 401 - 600, Frénicle n°s 601 - 880):
order-4 Frénicle magic square index 91
Extending this magic square we can construct a matrix of possibilities sized (2N-1)² to discover the other squares that are displayed on the magic torus:

order 4 Frénicle magic square index 91 expanded

This way we can find another 15 squares. We can see that there are 4 partially pandiagonal squares and 12 semi-magic squares displayed on the magic torus:

order 4 Frénicle index 91 converting to magic torus step 1

Then we can convert the squares that we have found into Frénicle standard form:

order 4 Frénicle index 91 converting to magic torus step 2

We can see that the magic torus displays not only the partially pandiagonal square Frénicle n°91, but also 3 other partially pandiagonal squares (Frénicle n°150, 320, and 394), as well as 12 semi-magic squares.

We are looking at the partially pandiagonal torus type n° T4.03.1.2. (index n° T4.174):

order 4 perspective partially pandiagonal magic torus index T4.174 type T4.03.1.2

Ironically, although the procedure is invaluable in eliminating doubles, conversion into Frénicle standard form (by rotation, transposition, and / or reflection) has until today concealed the toroidal continuity of magic squares.

In the end, magic squares are to be observed in only two fundamental ways: either from outside the torus or from within, and it does not really matter which, as the torus can always be turned inside out...
animation inside-out magic torus
Inside-out torus by Surot [Public domain], via Wikimedia Commons

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