# Divisibility sequence

In mathematics, a divisibility sequence is an integer sequence ${\displaystyle (a_{n})}$ indexed by positive integers n such that

${\displaystyle {\text{if }}m\mid n{\text{ then }}a_{m}\mid a_{n}}$

for all mn. That is, whenever one index is a multiple of another one, then the corresponding term also is a multiple of the other term. The concept can be generalized to sequences with values in any ring where the concept of divisibility is defined.

A strong divisibility sequence is an integer sequence ${\displaystyle (a_{n})}$ such that for all positive integers mn,

${\displaystyle \gcd(a_{m},a_{n})=a_{\gcd(m,n)}.}$

Every strong divisibility sequence is a divisibility sequence: if ${\displaystyle m\mid n}$ then ${\displaystyle \gcd(m,n)=m}$. Then by the strong divisibility property, ${\displaystyle \gcd(a_{m},a_{n})=a_{m}}$ and therefore ${\displaystyle a_{m}\mid a_{n}}$.

## Examples

• Any constant sequence is a strong divisibility sequence.
• Every sequence of the form ${\displaystyle a_{n}=kn}$, for some nonzero integer k, is a divisibility sequence.
• Every sequence of the form ${\displaystyle a_{n}=A^{n}-B^{n}}$ for integers ${\displaystyle A>B>0}$ is a divisibility sequence.
• The Fibonacci numbers Fn form a strong divisibility sequence.
• More generally, any Lucas sequence of the first kind Un(P,Q) is a divisibility sequence. Moreover, it is a strong divisibility sequence when gcd(P,Q)=1.
• Elliptic divisibility sequences are another class of such sequences.

## References

• Everest, Graham; van der Poorten, Alf; Shparlinski, Igor; Ward, Thomas (2003). Recurrence Sequences. American Mathematical Society. ISBN 978-0-8218-3387-2.
• Hall, Marshall (1936). "Divisibility sequences of third order". Am. J. Math. 58: 577–584. JSTOR 2370976.
• Ward, Morgan (1939). "A note on divisibility sequences". Bull. Amer. Math. Soc. 45 (4): 334–336. doi:10.1090/s0002-9904-1939-06980-2.
• Hoggatt, Jr., V. E.; Long, C. T. (1973). "Divisibility properties of generalized Fibonacci polynomials" (PDF). Fibonacci Quarterly: 113.
• Bézivin, J.-P.; Pethö, A.; van der Porten, A. J. (1990). "A full characterization of divisibility sequences". Am. J. Math. 112 (6): 985–1001. JSTOR 2374733.
• P. Ingram; J. H. Silverman (2012), "Primitive divisors in elliptic divisibility sequences", in Dorian Goldfeld; Jay Jorgenson; Peter Jones; Dinakar Ramakrishnan; Kenneth A. Ribet; John Tate, Number Theory, Analysis and Geometry. In Memory of Serge Lang, Springer, pp. 243–271, ISBN 978-1-4614-1259-5