how to build a magic square of any odd size with three rules
Built as a memo rather than a table: the thing worth hanging it up for reads across the room, with the full reference in smaller type below it.
Three sizes in one archive — from a sheet on the fridge to a poster on the wall.
A magic square holds every number from 1 to n² exactly once, and every row, every column and both diagonals add up to the same total.
The numbers 1 to 9 make exactly eight 3 × 3 magic squares — and all eight are the same square, turned or mirrored. Every one has 5 in the centre.
Why: the 4 lines through the centre add up to 4 × 15 = 60. That is all nine numbers (45) plus the centre counted three more times, so the centre is (60 − 45) ÷ 3 = 5.
Why the numbers can be trusted. Every countable number here is computed from data rather than typed in, and the sheet must pass 5 independent checks before it is allowed to build — how many items there are, the order they come in, and the claim in the highlighted box itself. If one fails, there is no file.
the IWF colour code and the plate count: from target weight to each side of the bar, no calculator
why a “yes” from a test that is right 99 times in 100 is wrong more than nine times in ten when one in a thousand has the thing
the first digit of a number: why a one turns up more than six times as often as a nine, and in which sets that is true
how many people it takes for two of them to share a birthday — and the same count for codes and fingerprints