Automatic sequence

An automatic sequence (or k-automatic sequence) is an infinite sequence of terms characterized by a finite automaton. The n-th term of the sequence is a mapping of the final state of the automaton when its input is the digits of n in some fixed base k. A k-automatic set is a set of non-negative integers for which the sequence of values of its characteristic function is an automatic sequence: that is, membership of n in the set can be determined by a finite state automaton on the digits of n in base k.

An automaton reading the base k digits from the most significant is said to be direct reading, and from the least significant is reverse reading. However the two directions lead to the same class of sequences.

Every automatic sequence is a morphic word.

Automaton point of view

Let k be a positive integer, and D = (E, φ, e) be a deterministic automaton where

• E is the finite set of states
• φ : E×[0,k − 1] → E is the transition function
• $e\in E$ is the initial state

also let A be a finite set, and π:E → A a projection towards A.

Extend the transition function φ from acting on single digits to acting on strings of digits by defining the action of φ on a string s consisting of digits s₁s₂...s_t as:

$\phi (e,s)=\phi (\phi (e,s_{1}s_{2}...s_{t-1}),s_{t})\,.$

Define a function m from the set of positive integers to the set A as follows:

$m(n)=\pi (\phi (e,s(n)))\,,$

where s(n) is n written in base k. Then the sequence m = m(1)m(2)m(3)... is called a k-automatic sequence.

Substitution point of view

Let σ be a k-uniform morphism of the free monoid E^∗, so that $\sigma (E)\subseteq E^{k}$ and which is prolongable on $e\in E$: that is, σ(e) begins with e. Let A and π be defined as above. Then if w is a fixpoint of σ, that is to say w = σ(w), then m = π(w) is a k-automatic sequence over A: this is Cobham's theorem. Conversely every k-automatic sequence is obtained in this way.

Decimation

Fix k > 1. For a sequence w we define the k-decimations of w for r=0,1,...,k-1 to be the subsequences consisting of the letters in positions congruent to r modulo k. The decimation kernel of w consists of the set of words obtained by all possible repeated decimations of w. A sequence is k-automatic if an only if the k-decimation kernel is finite.

1-automatic sequences

k-automatic sequences are normally only defined for k ≥ 2. The concept can be extended to k = 1 by defining a 1-automatic sequence to be a sequence whose n-th term depends on the unary notation for n, that is (1)ⁿ. Since a finite state automaton must eventually return to a previously visited state, all 1-automatic sequences are eventually periodic.

Properties

For given k and r, a set is k-automatic if and only if it is k^r-automatic. Otherwise, for h and k multiplicatively independent, then a set is both h-automatic and k-automatic if and only if it is 1-automatic, that is, ultimately periodic. This theorem is also due to Cobham, with a multidimensional generalisation due to Semënov.

If u(n) is a k-automatic sequence then the sequences u(kⁿ) and u(kⁿ−1) are ultimately periodic. Conversely, if v(n) is ultimately periodic then the sequence u defined by u(kⁿ) = v(n) and otherwise zero is k-automatic.

Let u(n) be a k-automatic sequence over the alphabet A. If f is a uniform morphism from A^∗ to B^∗ then the word f(u) is k-automatic sequence over the alphabet B.

Let u(n) be a sequence over the alphabet A and suppose that there is an injective function j from A to the finite field F_q. The associated formal power series is

$f_{u}(z)=\sum _{n}j(u(n))z^{n}\ .$

The sequence u is q-automatic if and only if the power series f_u is algebraic over the rational function field F_q(z).

Examples

The following sequences are automatic:

• Thue-Morse sequence: take E = A = {0, 1}, e = 0, π = id, and σ such that σ(0) = 01, σ(1) = 10; we get the fixpoint 01101001100101101001011001101001..., which is in fact the Thue-Morse word. The n-th term is the parity of the base 2 representation of n and the sequence is thus 2-automatic. The 2-kernel consists of the sequence itself and its complement. The associated power series T(z) satisfies

$(1+z)^{3}T^{2}+(1+z)^{2}T+z=0\$

over the field F₂(z).

• Rudin–Shapiro sequence
• Baum–Sweet sequence
• Regular paperfolding sequence and a general paperfolding sequence with a periodic sequence of folds
• The period-doubling sequence, defined by the parity of the power of 2 dividing n; it is the fixed point of the morphism 0 → 01, 1 → 00.

Automatic real number

An automatic real number is a real number for which the base-b expansion is an automatic sequence. All such numbers are either rational or transcendental, but not a U-number. Rational numbers are k-automatic in base b for all k and b.

See also

• Büchi arithmetic

References

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