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A242590 Numbers whose representation in Roman numerals is horizontally symmetrical.

%I A242590 #46 Sep 14 2017 09:45:47
%S A242590 1,2,3,9,10,11,12,13,19,20,21,22,23,29,30,31,32,33,39,90,91,92,93,99,
%T A242590 100,101,102,103,109,110,111,112,113,119,120,121,122,123,129,130,131,
%U A242590 132,133,139,190,191,192,193,199,200,201,202,203,209,210,211,212,213,219,220,221,222,223,229,230,231,232,233,239,290
%N A242590 Numbers whose representation in Roman numerals is horizontally symmetrical.
%C A242590 The sequence contains only 224 terms and ends with 899.
%C A242590 Roman numerals use the letters I, V, X, L, C, D and M, combinations of which are used to represent numbers. Since the letters V, L and M are not horizontally symmetrical, numbers containing these letters are not part of the sequence. Importantly, Roman numerals for 900 and beyond will always contain the numeral M, so the sequence ends at 899.
%H A242590 Philip Mizzi, <a href="/A242590/b242590.txt">Table of n, a(n) for n = 1..224</a>
%e A242590 Define two functions:
%e A242590 R(n) converts the number n to Roman number notation.
%e A242590 H[x] takes the argument x and produces a truth value, determining if the argument is horizontally symmetrical.
%e A242590 Hence,
%e A242590 for n = 1, R(n) = I, H[R(n)] = TRUE, so n = 1 is a term,
%e A242590 for n = 5, R(n) = V, H[R(n)] = FALSE, so n = 5 is not a term;
%e A242590 for n = 11, R(n) = XI, H[R(n)] = TRUE, so n = 11 is a term;
%e A242590 for n = 50, R(n) = L, H[R(n)] = FALSE, so n = 50 is not a term;
%e A242590 for n = 100, R(n) = C, H[R(n)] = TRUE, so n = 100 is a term;
%e A242590 for n = 900, R(n) = CM, H[R(n)] = FALSE, so n = 900 is not a term;
%e A242590 for n = 1000, R(n) = M, H[R(n)] = FALSE, so n = 1000 is not a term.
%Y A242590 Cf. A007284 (horizontally/Arabic), A166874 (vertically/Roman).
%K A242590 nonn,base,fini,full
%O A242590 1,2
%A A242590 _Philip Mizzi_, May 24 2014
%E A242590 Name edited by _Jon E. Schoenfield_, Sep 12 2017