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1, 3 and 9
They're all multiples of 9.
18, 27, 36, 45, 54, 63.
9, 18, 27, 36, 45, 54, 63, 72
15, 30, 45, 60, 75, 90... 9, 18, 27, 36, 45, 54, 63...
45 - 63 = -18