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 A387014 #52 Aug 22 2025 11:02:40
%S A387014 1,2,3,4,9,5,6,7,8,10,11,12,13,14,19,15,16,17,18,20,18,17,16,15,48,47,
%T A387014 46,45,49,44,43,42,41,40,19,14,13,12,11,10,8,7,6,5,88,5,6,7,8,10,8,7,
%U A387014 6,5,88,87,86,85,89,84,83,82,81,80,78,77,76,75,79,74,73,72,71,70
%N A387014 Middle element of the list {1, 2, 3, ..., 2n-1} when elements are sorted lexicographically by their Roman numeral representations.
%C A387014 Usual Roman numeral representations are taken as ending at 3999 so that here n <= 2000.
%C A387014 a(21) = 18 is the first repeated element, and 21 is also the least positive integer not in this sequence, which has 1690 distinct terms. More exhaustively, the positive integers not in this sequence are 21 .. 39, 90 .. 100, 200 .. 236, 239 .. 399, 450 .. 484, 489, 900 .. 999, 1005 .. 1008, 1010 .. 1049, 1090 .. 1099, 2001 .. 2099, 2101 .. 2199, 2205 .. 2208, 2210 .. 2249, 2251 .. 2299 and 2400 .. oo (where a .. b means all integers from a to b). - _M. F. Hasler_, Aug 18 2025
%H A387014 José Hernández, <a href="/A387014/b387014.txt">Table of n, a(n) for n = 1..2000</a>
%e A387014 For n = 5, the 2n-1 list elements sorted by Roman numerals are I, II, III, IV, IX, V, VI, VII, VIII and the middle element is IX = 9 = a(5).
%t A387014 Table[SortBy[Range[2*n-1], RomanNumeral][[n]], {n, 100}] (* Misha Lavrov via SeqFan *)
%o A387014 (Python)
%o A387014 import roman
%o A387014 def A387014(n):
%o A387014 return roman.fromRoman(sorted(roman.toRoman(k)for k in range(1,2*n))[n-1]) # _M. F. Hasler_, Aug 18 2025
%Y A387014 Cf. A036742.
%K A387014 base,fini,full,nonn,new
%O A387014 1,2
%A A387014 _José Hernández_, Aug 13 2025