Hellenica World

# Integer sequence

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.

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
Home primes
Hyperperfect numbers
Juggler sequence
Kolakoski sequence
Lucky numbers
Lucas 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.
Complete sequences

An integer sequence is called a complete sequence if every positive integer can be expressed as a sum of values in the sequence, using each value at most once.

On-Line Encyclopedia of Integer Sequences