Foias constant

In mathematical analysis, the Foias constant is a real number named after Ciprian Foias.

Evolution of the sequence ${\displaystyle x_{n+1}=(1+1/x_{n})^{n}}$ for several values of ${\displaystyle x_{1}}$, around the Foias constant. All of them lead to two accumulation points, viz. 1 and ${\displaystyle \infty }$. A logarithmic scale is used.

It is defined in the following way: for every real number x₁ > 0, there is a sequence defined by the recurrence relation

${\displaystyle x_{n+1}=\left(1+{\frac {1}{x_{n}}}\right)^{n}}$

for n = 1, 2, 3, .... The Foias constant is the unique choice α such that if x₁ = α then the sequence diverges to infinity.[1] Numerically, it is

${\displaystyle \alpha =1.187452351126501\ldots }$ OEIS: A085848.

No closed form for the constant is known.

When x₁ = α then we have the limit:

${\displaystyle \lim _{n\to \infty }x_{n}{\frac {\log n}{n}}=1,}$

where "log" denotes the natural logarithm. Consequently, one has by the prime number theorem that in this case

${\displaystyle \lim _{n\to \infty }{\frac {x_{n}}{\pi (n)}}=1,}$

where π is the prime-counting function.[1]

See also

• Mathematical constant

Notes and references

1. Ewing, J. and Foias, C. "An Interesting Serendipitous Real Number." In Finite versus Infinite: Contributions to an Eternal Dilemma (Ed. C. Caluse and G. Păun). London: Springer-Verlag, pp. 119–126, 2000.

• Weisstein, Eric W. "Foias Constant". MathWorld.
• S. R. Finch (2003). Mathematical Constants. Cambridge University Press. p. 430. ISBN 0-521-818-052. Foias constant.
• Sloane, N. J. A. (ed.). "Sequence A085848 (Decimal expansion of Foias's Constant)". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation.