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 A309681 Lexically least sequence such that a(n) next appears at index n+a(n+1) and such that a(1)=2. 5
 2, 4, 5, 6, 2, 6, 4, 13, 5, 6, 2, 6, 13, 9, 5, 18, 2, 18, 9, 4, 19, 13, 9, 17, 6, 20, 18, 26, 30, 17, 13, 35, 5, 19, 2, 19, 35, 23, 4, 9, 47, 23, 17, 20, 6, 30, 50, 4, 60, 20, 50, 5, 18, 2, 18, 50, 41, 26, 23, 35, 7, 42, 17, 47, 21, 13, 35, 37, 60, 5, 19, 2, 19, 60, 6 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS This sequence is aperiodic. Proof: If any number of initial terms are removed, said initial terms can be recreated by extrapolating backwards from later terms. In other words, the later terms include a perfect description of the earlier terms. This implies that, if the sequence is periodic, the repeating section must include all of the initial terms of the sequence in the order in which they originally appear. This is impossible because we cannot extrapolate backwards infinitely from the initial terms. If we attempt to do so, we get the pattern 5, 5, 4, 2, 4, 5 (the last three values being the first three terms of the sequence). This pattern is clearly invalid, so the sequence must be aperiodic. - Samuel B. Reid, Jun 07 2020 LINKS Samuel B. Reid, Table of n, a(n) for n = 1..10000 Samuel B. Reid, Python program for A309681 Samuel B. Reid, C program for A309681 EXAMPLE If a(2) were 1, a(2) would have to be 2. a(2) cannot be both 1 and 2, so a(2) cannot be 1. If a(2) were 2, the next 2 after a(1) would appear at a(3). The next 2 actually appears at a(2), so a(2) cannot be 2. If a(2) were 3, a(4) would have to be 2. If a(4) were 2, a(3) and a(5) would have to have the same value. If a(3) and a(5) were equal, the pattern X, 3, Y, 2 would appear later in the sequence. The pattern X, 3, Y, 2 is invalid because X would have to be 2. X cannot be 2 because it comes after the 2 at a(1) and before the 2 that succeeds Y. This behavior is inconsistent with the definition of the sequence. The next possible value for a(2) is 4, which causes no trivial inconsistencies (as with a(2)=1 or a(2)=2) or logical inconsistencies (as with a(2)=3). The n-th term of the sequence is the lowest value that does not cause inconsistencies of either sort. PROG (Python and C) See Links section. CROSSREFS Cf. A171922. Sequence in context: A138083 A181524 A240568 * A287224 A077867 A123124 Adjacent sequences:  A309678 A309679 A309680 * A309682 A309683 A309684 KEYWORD nonn AUTHOR Samuel B. Reid, Aug 12 2019 STATUS approved

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Last modified April 20 23:46 EDT 2021. Contains 343143 sequences. (Running on oeis4.)