A107604 Irregular triangle read by rows of the positions of 2's in the triangle generated by the morphism 1->12, 2->1.

2, 2, 2, 2, 5, 2, 5, 2, 5, 7, 2, 5, 7, 2, 5, 7, 2, 5, 7, 10, 2, 5, 7, 10, 2, 5, 7, 10, 2, 5, 7, 10, 13, 2, 5, 7, 10, 13, 2, 5, 7, 10, 13, 15, 2, 5, 7, 10, 13, 15, 2, 5, 7, 10, 13, 15, 2, 5, 7, 10, 13, 15, 18, 2, 5, 7, 10, 13, 15, 18, 2, 5, 7, 10, 13, 15, 18, 20

Offset

2,1

Links

Table of n, a(n) for n=2..79.

Sean A. Irvine, Java program (github)

Example

Begins:
  row triangle              positions of 2's
   2  12(1)              -> 2;
   3  121(12)            -> 2;
   4  1211(212)          -> 2;
   5  12112(121)         -> 2, 5;
   6  121121(2112)       -> 2, 5;
   7  1211212(1121)      -> 2, 5, 7;
   8  12112121(12112)    -> 2, 5, 7;
   9  121121211(211212)  -> 2, 5, 7;
  10  1211212112(112121) -> 2, 5, 7, 10;
at each row we expand one more symbol and report the positions of 2 that are <= row number.

Mathematica

s[1] = {1, 2}; s[2] = {1};; t[a_] := Flatten[s /@ a]; p[0] = {1}; p[1] = t[p[0]]; p[n_] := t[p[n - 1]] a = Table[Flatten[Table[If[p[i][[j]] == 2, j, {}], {j, 1, i}]], {i, 1, 20}]

Crossrefs

Cf. A107638.

Sequence in context: A154097 A221491 A224254 * A080647 A324516 A181058

Adjacent sequences: A107601 A107602 A107603 * A107605 A107606 A107607

Keyword

nonn,tabf,changed

Author

Roger L. Bagula, Jun 09 2005

Extensions

Edited by Sean A. Irvine, Jun 23 2026

Status

approved