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 A075421 Trajectory of n under the Reverse and Add! operation carried out in base 4 (presumably) does not reach a palindrome and (presumably) does not join the trajectory of any term m < n. 16
 290, 318, 719, 795, 799, 1210, 3903, 4199, 4207, 4219, 4236, 4278, 4279, 4294, 4326, 4333, 4334, 4338, 4402, 4598, 4662, 4726, 5046, 5357, 6157, 6174, 7246, 7247, 7295, 7407, 7549, 8063, 8191, 9211, 12319, 12431, 12463, 12539, 15487, 16519, 16587 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS For 318 (cf. A075153), 266718 (cf. A075466) and 270798 (cf. A075467) one can prove that the base 4 trajectory does not contain a palindrome. A proof for 290 (cf. A075299) has not been found up to now. 4398859679359 is another known candidate (obtained from a remark of David J. Seal, cf. Links) for a term whose trajectory is provably palindrome-free, but is not secured that it does not join the trajectory of some term m < n. - If the trajectory of an integer k joins the trajectory of a smaller integer which is a term of the present sequence, then this occurs after very few Reverse and Add! steps (at most 28 for k < 20000). On the other hand, the trajectories of the terms listed above do not join the trajectory of any smaller term within at least 1000 steps. Base-4 analog of A063048 (base 10) and A075252 (base 2); subsequence of A075420. From A.H.M. Smeets, Mar 18 2019: (Start) David J. Seal (see LINKS) observed a cyclic pattern (length 6) in the trajectories that can be represented by an extended right regular grammar with production rules: S -> S_a | S_b | S_c | S_d | S_e | S_f, S_a -> 1033202000232 T_a, T_a -> 222 T_a | 2302333113230 S_b -> 2022321332331 T_b, T_b -> 111 T_b | 1223001203131 S_c -> 10002003002212 T_c, T_c -> 222 T_c | 3221333101333 S_d -> 103312202321111 T_d, T_d -> 111 T_d | 1102023122000 S_e -> 110200123122222 T_e, T_e -> 222 T_e | 2231232001301 S_f -> 213301021321111 T_f, T_f -> 111 T_f | 1113213003312 Within the first 471 terms of this sequence we observed three trajectories with a cyclic pattern (length 6) that can be represented by a context-free grammar with production rules: S -> S_a | S_b | S_c | S_d | S_e | S_f, S_a -> 10 T_a 00, T_a -> 3 T_a 0 | T_a0, S_b -> 11 T_b 01, T_b -> 0 T_b 3 | T_b0, S_c -> 22 T_c 12, T_c -> 0 T_c 3 | T_c0, S_d -> 10 T_d 000, T_d -> 3 T_d 0 | T_d0, S_e -> 11 T_e 301, T_e -> 0 T_e 3 | T_e0, S_f -> 22 T_f 312, T_f -> 0 T_f 3 | T_f0. The terminating strings in these context-free grammars are given by: n     2      359                371 a(n)  318    266718             270798 T_a0  33230  33230000001033230  3323001033230 T_b0  03123  03123010001103123  0312302103123 T_c0  01313  01313120002201313  0131320201313 T_d0  33323  33323000001033323  3332300103323 T_e0  03222  03222301001103222  0322201113222 T_f0  02111  02111312002202111  0211112222111 From the fact that both, right regular grammars and context-free grammars occur, we wonder if other trajectories can be represented by context-sensitive grammars as well, by which other trajectories can be proven never to end up in a palindromic string? (End) LINKS A.H.M. Smeets, Table of n, a(n) for n = 1..471 Klaus Brockhaus, Illustration: Distribution of terms below 2000000 David J. Seal, Results EXAMPLE 719 is a term since the trajectory of 719 (presumably) does not lead to an integer which occurs in the trajectory of 290 or of 318. MATHEMATICA limit = 10^3; (* Assumes that there is no palindrome if none is found before "limit" iterations *) utraj = {}; Select[Range[0, 17000], (x = NestWhileList[# + IntegerReverse[#, 4] &, #, # !=IntegerReverse[#, 4] & , 1, limit];    If[Length[x] >= limit  && Intersection[x, utraj] == {},     utraj = Union[utraj, x]; True, utraj = Union[utraj, x]]) &] (* Robert Price, Oct 16 2019 *) CROSSREFS Cf. A063048, A075252, A075420, A075299, A075153, A075466, A075467, A091675. Sequence in context: A129245 A186553 A075420 * A332229 A296055 A090839 Adjacent sequences:  A075418 A075419 A075420 * A075422 A075423 A075424 KEYWORD base,nonn AUTHOR Klaus Brockhaus, Sep 18 2002, revised Jan 28 2004 STATUS approved

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Last modified September 23 20:42 EDT 2021. Contains 347617 sequences. (Running on oeis4.)