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 A308172 #11 May 17 2019 08:14:43 %S A308172 1,2,3,3,3,5,5,5,6,6,6,9,9,9,10,10,10,11,11,11,13,13,13,15,15,15,17, %T A308172 17,17,18,18,18,19,19,19,22,22,22,23,23,24,24,24,26,26,26,29,29,29,30, %U A308172 30,30,31,31,31,32,32,32,35,35,35,36,36,36,38,38,38,39,39,39,40,40,43 %N A308172 a(0)=1; thereafter a(n) = smallest number missing from A081145 after A081145(n) has been calculated. %C A308172 More than the usual number of terms are shown in order to emphasize the irregularities. %C A308172 Usually the terms appear in groups of three, but there are many irregularities. To see them, take this sequence, apply the RUNS transform twice, and bisect the result. This produces a very irregular sequence, 12, 10, 1, 46, 2, 2, 3, 2, 5, 6, 7, 10, 9, 13, 25, 5, 24, 1, 42, ... It would be nice to understand this better. %H A308172 Rémy Sigrist, <a href="/A308172/b308172.txt">Table of n, a(n) for n = 0..25000</a> %H A308172 Rémy Sigrist, <a href="/A308172/a308172.gp.txt">PARI program for A308172</a> %e A308172 n=4: Terms 1 through 4 of A081145 are 1,2,4,7. After A081145(4)=7 has been calculated, the numbers not yet in A081145 are 3,5,6,8,..., so a(4) = 3. %o A308172 (PARI) See Links section. %Y A308172 Cf. A081145. %K A308172 nonn %O A308172 0,2 %A A308172 _N. J. A. Sloane_, May 16 2019