What common property unites all of the following numbers?
4, 9, 60, 101, 400, 450, 910, 1010
The set of numbers that share this common property is not limited to just these eight, but the list should provide enough information to solve it.
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I guess it is
Roman numerals with 1 letter, in alphabetical order; then those with 2 letters, etc.
and the source is http://oeis.org/A003588