No, 18 is not a factor of 81.
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The greatest common factor of 36, 81 and 18 is 9
9, 18, 27, 36, 45, 54, 63, 72, 81, 90, 99.
9, 18, 27, 36, 45, 54, 63, 72, 81, 90, 99 and on to infinity.
9, 18, 27, 36, 45, 54, 63, 72, 81, 90, 99, 108 . . .
36 and 45 or 9 and any multiple of 9