9
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9 is the third square number (9 = 32)
9 divides into a number if the sum of the digits is divisible by 9
9 is the only square that is the sum of two consecutive cubes: 9= 13 + 23
The first 9 numbers can be arranged into a "magic" square so that all rows, columns and diagonals add to 15. There are 8 different ways this can be done. |