In mathematics, an integer sequence is a sequence (i.e., an ordered list) of integers.
An integer sequence may be specified explicitly by giving a formula for its nth term, or implicitly by giving a relationship between its terms. For example, the sequence 0, 1, 1, 2, 3, 5, 8, 13, … (the Fibonacci sequence) is formed by starting with 0 and 1 and then adding any two consecutive terms to obtain the next one: an implicit description. The sequence 0, 3, 8, 15, … is formed according to the formula n2 − 1 for the nth term: an explicit definition.
Alternatively, an integer sequence may be defined by a property which members of the sequence possess and other integers do not possess. For example, we can determine whether a given integer is a perfect number, even though we do not have a formula for the nth perfect number.
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Examples
Integer sequences which have received their own name include:
- Abundant numbers
- Baum–Sweet sequence
- Bell numbers
- Binomial coefficients
- Carmichael numbers
- Catalan numbers
- Composite numbers
- Deficient numbers
- Euler numbers
- Even and odd numbers
- Factorial numbers
- Fibonacci numbers
- Fibonacci word
- Figurate numbers
- Golomb sequence
- Happy numbers
- Highly totient numbers
- Highly composite numbers
- Hyperperfect numbers
- Juggler sequence
- Kolakoski sequence
- Lucky numbers
- Lucas numbers
- Padovan numbers
- Partition numbers
- Perfect numbers
- Pseudoperfect numbers
- Prime numbers
- Pseudoprime numbers
- Regular paperfolding sequence
- Rudin–Shapiro sequence
- Semiperfect numbers
- Semiprime numbers
- Superperfect numbers
- Thue-Morse sequence
- Ulam numbers
- Weird numbers
Computable and definable sequences
An integer sequence is a computable sequence, if there exists an algorithm which given n, calculates an, for all n > 0. An integer sequence is a definable sequence, if there exists some statement P(x) which is true for that integer sequence x and false for all other integer sequences. The set of computable integer sequences and definable integer sequences are both countable, with the computable sequences a proper subset of the definable sequences (in other words, some sequences are definable but not computable). The set of all integer sequences is uncountable (with cardinality equal to that of the continuum); thus, almost all integer sequences are uncomputable and cannot be defined.
See also
External links
- Journal of Integer Sequences. Articles are freely available online.
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