# Feller's coin-tossing constants

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Feller's coin-tossing constants are a set of numerical constants which describe asymptotic probabilities that in n independent tosses of a fair coin, no run of k consecutive heads (or, equally, tails) appears.

William Feller showed that if this probability is written as p(n,k) then

$\lim _{n\rightarrow \infty }p(n,k)\alpha _{k}^{n+1}=\beta _{k}$ where αk is the smallest positive real root of

$x^{k+1}=2^{k+1}(x-1)$ and

$\beta _{k}={2-\alpha _{k} \over k+1-k\alpha _{k}}.$ ## Values of the constants

k $\alpha _{k}$ $\beta _{k}$ 1 2 2
2 1.23606797... 1.44721359...
3 1.08737802... 1.23683983...
4 1.03758012... 1.13268577...

For $k=2$ the constants are related to the golden ratio, $\varphi$ , and Fibonacci numbers; the constants are ${\sqrt {5}}-1=2\varphi -2=2/\varphi$ and $1+1/{\sqrt {5}}$ . The exact probability p(n,2) can be calculated either by using Fibonacci numbers, p(n,2) = ${\tfrac {F_{n+2}}{2^{n}}}$ or by solving a direct recurrence relation leading to the same result. For higher values of $k$ , the constants are related to generalizations of Fibonacci numbers such as the tribonacci and tetranacci numbers. The corresponding exact probabilities can be calculated as p(n,k) = ${\tfrac {F_{n+2}^{(k)}}{2^{n}}}$ . 

## Example

If we toss a fair coin ten times then the exact probability that no pair of heads come up in succession (i.e. n = 10 and k = 2) is p(10,2) = ${\tfrac {9}{64}}$ = 0.140625. The approximation $p(n,k)\approx \beta _{k}/\alpha _{k}^{n+1}$ gives 1.44721356...×1.23606797...−11 = 0.1406263...