OFFSET
1
COMMENTS
This sequence appears to be: 0, followed by A080846. [R. J. Mathar, May 16 2011]
From Michel Dekking, Oct 26 2019: (Start)
Proof of Mathar's third conjecture: Let alpha be the defining morphism for (a(n)): alpha(0)=001, alpha(1)=101. Let beta be the defining morphism for A080846: beta(0)=010, beta(1)=011.
To prove the conjecture, it suffices to prove that
alpha^n(0) 0 = 0 beta^n(0) for all n>0.
This holds for n=1. The important property of the morphisms alpha and beta is that
10^{-1}alpha^n(0) = alpha^n(1) and beta^n(0) = beta^n(1) 1^{-1}0 for all n>0.
Here 0^{-1} and 1^{-1} are the free group inverses of 0 and 1.
We use this in the induction step:
alpha^{n+1}(0) 0 = alpha^n(00)alpha^n(1) 0 =
alpha^n(0) alpha^n(0) 10^{-1}alpha^n(0) 0 =
0 beta^n(0) 0^{-1} 0 beta^n(0) 0^{-1} 10^{-1} 0 beta^n(0) 0^{-1} 0 =
0 beta^n(0) beta^n(1)1^{-1}0 0^{-1} 1 beta^n(0) =
0 beta^n(010) = 0 beta^{n+1}(0).
(End)
From Michel Dekking, Oct 26 2019: (Start)
Let beta be the defining morphism for this sequence. It is notationally convenient to write the morphism beta on the alphabet {A,B}:
beta: A -> AAB, B -> BAB.
Let delta be the 'decoration' morphism defined by delta(A) = 0, delta(B) = 10.
Let x = AABAABBABA... be the fixed point of beta.
CLAIM: delta(x) = A049320, the non-primitive Chacon sequence.
This is proved in Ferenczi, 1995. Here in an independent, short proof.
Let gamma given by 0->0010, 1->1 be the Chacon morphism, and let
c = 0010001010010... be the fixed point of gamma. One verifies that
delta(AAB) = delta(AABAABBAB) = gamma(0010),
delta(BAB) = delta(BABAABBAB) = gamma(10010).
So delta(beta(A)) = gamma(delta(A)), and delta(beta(B)) = gamma(delta(B)).
It follows that
delta(beta^n(A)) = gamma^n(delta(A)) = gamma^n(0) for all n>0.
The left side converges to the delta image of the fixed point x of beta, the right side to c, the non-primitive Chacon sequence.
We have shown that the [10->1]-transform of A049320 equals (a(n)).
(End)
A generalized choral sequence c(3n+r_0)=0, c(3n+r_1)=1, c(3n+r_c)=c(n), with r_0=1, r_1=2, r_c=0, and c(0)=0. - Joel Reyes Noche, Jun 14 2021
REFERENCES
Joel Reyes Noche, Generalized Choral Sequences, Matimyas Matematika, 31(2008), 25-28.
LINKS
S. Ferenczi, Les transformations de Chacon: combinatoire, structure géométrique, lien avec les systèmes de complexité 2n + 1, Bull. Soc. Math. France 123 (1995), 271-292.
Joel Reyes Noche, On generalized choral sequences, Gibon, IX(2011), 51-69.
FORMULA
a(3k-2)=a(k), a(3k-1)=0, a(3k)=1 for k>=1, a(0)=0.
EXAMPLE
Start: 0
Rules:
0 --> 001
1 --> 101
-------------
0: (#=1)
0
1: (#=3)
001
2: (#=9)
001001101
3: (#=27)
001001101001001101101001101
4: (#=81)
001001101001001101101001101001001101001001101101001101101001101001001101101001101
MATHEMATICA
PROG
(PARI) a(n) = my(r); if(n--, until(r, [n, r]=divrem(n, 3))); r==2; \\ Kevin Ryde, Nov 09 2021
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Clark Kimberling, Apr 24 2011
STATUS
approved