A107641 Irregular triangle read by rows: positions of 3's in the triangle generated by the morphism 1->2, 2->3, 3->123 starting from 2.

1, 3, 2, 1, 4, 3, 5, 2, 5, 6, 1, 4, 6, 3, 5, 8, 9, 2, 5, 6, 9, 1, 4, 6, 9, 3, 5, 8, 9, 12, 2, 5, 6, 9, 11, 1, 4, 6, 9, 12, 14, 3, 5, 8, 9, 12, 14, 2, 5, 6, 9, 11, 14, 1, 4, 6, 9, 12, 14, 17, 3, 5, 8, 9, 12, 14, 17, 2, 5, 6, 9, 11, 14, 17, 19, 1, 4, 6, 9, 12, 14, 17, 18

Offset

2,2

Links

Table of n, a(n) for n=2..84.

Example

Begins:
  row triangle    positions of 3's
   1  2        -> none;
   2  3        -> 1;
   3  123      -> 3;
   4  2312     -> 2;
   5  31232    -> 1, 4;
   6  123231   -> 3, 5;
   7  2312331  -> 2, 5, 6;
   8  31232312 -> 1, 4, 6;

Mathematica

w = {1}; Rest@ Reap[Do[w = PadRight[Flatten[Replace[w, {1 -> 2, 2 -> 3, 3 -> {1, 2, 3} }, 2] ], i]; Sow[Position[w, 3][[All, 1]] ], {i, 20}] ][[-1, 1]] // Flatten (* Michael De Vlieger, Jun 30 2026 *)

Crossrefs

Cf. A107640.

Sequence in context: A327467 A347832 A077427 * A299352 A127671 A271724

Adjacent sequences: A107638 A107639 A107640 * A107642 A107643 A107644

Keyword

nonn,tabf,changed

Author

Roger L. Bagula, Jun 09 2005

Extensions

Edited by Sean A. Irvine, Jun 28 2026

Offset corrected by Jianing Song, Jul 02 2026

Status

approved