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 A242590 Numbers whose representation in Roman numerals is horizontally symmetrical. 1
 1, 2, 3, 9, 10, 11, 12, 13, 19, 20, 21, 22, 23, 29, 30, 31, 32, 33, 39, 90, 91, 92, 93, 99, 100, 101, 102, 103, 109, 110, 111, 112, 113, 119, 120, 121, 122, 123, 129, 130, 131, 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 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS The sequence contains only 224 terms and ends with 899. 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. LINKS Philip Mizzi, Table of n, a(n) for n = 1..224 EXAMPLE Define two functions: R(n) converts the number n to Roman number notation. H[x] takes the argument x and produces a truth value, determining if the argument is horizontally symmetrical. Hence, for n = 1, R(n) = I, H[R(n)] = TRUE, so n = 1 is a term, for n = 5, R(n) = V, H[R(n)] = FALSE, so n = 5 is not a term; for n = 11, R(n) = XI, H[R(n)] = TRUE, so n = 11 is a term; for n = 50, R(n) = L, H[R(n)] = FALSE, so n = 50 is not a term; for n = 100, R(n) = C, H[R(n)] = TRUE, so n = 100 is a term; for n = 900, R(n) = CM, H[R(n)] = FALSE, so n = 900 is not a term; for n = 1000, R(n) = M, H[R(n)] = FALSE, so n = 1000 is not a term. CROSSREFS Cf. A007284 (horizontally/Arabic), A166874 (vertically/Roman). Sequence in context: A353653 A351650 A235499 * A135204 A037463 A309348 Adjacent sequences: A242587 A242588 A242589 * A242591 A242592 A242593 KEYWORD nonn,base,fini,full AUTHOR Philip Mizzi, May 24 2014 EXTENSIONS Name edited by Jon E. Schoenfield, Sep 12 2017 STATUS approved

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Last modified November 29 09:34 EST 2022. Contains 358423 sequences. (Running on oeis4.)