A166874 Numbers with vertical symmetry when expressed as Roman numerals.
1, 2, 3, 5, 10, 19, 20, 30, 1000, 2000, 3000
Offset: 1
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Edited by N. J. A. Sloane, Oct 25 2009
This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.