They are powers of 3.
729 = 36
243 = 35
81 = 34
27 = 33
9 = 32
3 = 31
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243, 81, 9, 3
The cube root of 729 is 9 and 9 squared is 81.
81
The list could go on forever, but here are some basic multiples. 27, 54, 81, 108, 135, ....
3 3^5 = 243 (3 x 3) x (3 x 3) x 3 = 243 9 x 9 x 3 = 243 81 x 3 = 243