Golomb sequence

In mathematics, the Golomb sequence, named after Solomon W. Golomb (but also called Silverman's sequence), is a monotonically increasing integer sequence where a_n is the number of times that n occurs in the sequence, starting with a₁ = 1, and with the property that for n > 1 each a_n is the smallest positive integer which makes it possible to satisfy the condition. For example, a₁ = 1 says that 1 only occurs once in the sequence, so a₂ cannot be 1 too, but it can be 2, and therefore must be 2. The first few values are

1, 2, 2, 3, 3, 4, 4, 4, 5, 5, 5, 6, 6, 6, 6, 7, 7, 7, 7, 8, 8, 8, 8, 9, 9, 9, 9, 9, 10, 10, 10, 10, 10, 11, 11, 11, 11, 11, 12, 12, 12, 12, 12, 12 (sequence A001462 in the OEIS).

a₁ = 1
Therefore, 1 occurs exactly one time in this sequence.

a₂ > 1
a₂ = 2

2 occurs exactly 2 times in this sequence.
a₃ = 2

3 occurs exactly 2 times in this sequence.

a₄ = a₅ = 3

4 occurs exactly 3 times in this sequence.
5 occurs exactly 3 times in this sequence.

a₆ = a₇ = a₈ = 4
a₉ = a₁₀ = a₁₁ = 5

etc.

Colin Mallows has given an explicit recurrence relation ${\displaystyle a(1)=1;a(n+1)=1+a(n+1-a(a(n)))}$. An asymptotic expression for a_n is

${\displaystyle \varphi ^{2-\varphi }n^{\varphi -1},}$

where ${\displaystyle \varphi }$ is the golden ratio (approximately equal to 1.618034).

• Everest, Graham; van der Poorten, Alf; Shparlinski, Igor; Ward, Thomas (2003). Recurrence sequences. Mathematical Surveys and Monographs. Vol. 104. Providence, RI: American Mathematical Society. pp. 10, 256. ISBN 0-8218-3387-1. Zbl 1033.11006.
• Guy, Richard K. (2004). Unsolved problems in number theory (3rd ed.). Springer-Verlag. Section E25. ISBN 0-387-20860-7. Zbl 1058.11001.

• Python code for Golomb Sequence