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.
%I A215121 #17 Feb 11 2020 09:49:17 %S A215121 1,2,3,10,19,20,30 %N A215121 Strobogrammatic palindromic numbers in their Roman numeral representation. %C A215121 Numbers which, written as Roman numerals, are the same upside down and backwards. %C A215121 Upside-down-invariant numbers are also called ambigrams. "Upside down" here means rotated by 180 degrees (i.e., central symmetry), NOT "vertically flipped" (symmetry w.r.t. horizontal line). %C A215121 V, L, C, D, M (5, 50, 100, 500, 1000 in decimal) are not the same upside-down. Excludes "old style" Roman numeral representation of n (e.g., IIII rather than IV). %e A215121 I, II, III, X, XIX, XX, XXX. %Y A215121 Cf. A000787, A006968, A078715, A166874. %K A215121 nonn,base,fini,full %O A215121 1,2 %A A215121 _Jonathan Vos Post_, Aug 03 2012