Fibonorial

In mathematics, the Fibonorial n!_F, also called the Fibonacci factorial, where n is a nonnegative integer, is defined as the product of the first n positive Fibonacci numbers, i.e.

${n!}_F:=\prod _{i=1}^n{F}_i,n\ge 0,$

where F_i is the i^th Fibonacci number, and 0!_F gives the empty product (defined as the multiplicative identity, i.e. 1).

The Fibonorial n!_F is defined analogously to the factorial n!. The Fibonorial numbers are used in the definition of Fibonomial coefficients (or Fibonacci-binomial coefficients) similarly as the factorial numbers are used in the definition of binomial coefficients.

Asymptotic behaviour

The series of fibonorials is asymptotic to a function of the golden ratio $\phi$: $n{!}_F\sim C\frac{{\phi }^{n(n+1)/2}}{5^{n/2}}$.

Here the fibonorial constant (also called the fibonacci factorial constant) $C$ is defined by $C=\prod _{k=1}^{\mathrm{\infty }}(1-a^k)$, where $a=-\frac{1}{{\phi }^2}$ and $\phi$ is the golden ratio.

An approximate truncated value of $C$ is 1.226742010720 (see (sequence A062073 in the OEIS) for more digits).

Almost-Fibonorial numbers

Almost-Fibonorial numbers: n!_F − 1.

Almost-Fibonorial primes: prime numbers among the almost-Fibonorial numbers.

Quasi-Fibonorial numbers

Quasi-Fibonorial numbers: n!_F + 1.

Quasi-Fibonorial primes: prime numbers among the quasi-Fibonorial numbers.

Connection with the q-Factorial

The fibonorial can be expressed in terms of the q-factorial and the golden ratio $\phi =\frac{1+\sqrt{5}}{2}$:

$n{!}_F={\phi }^{(\genfrac{}{}{0ex}{}{n}{2})}[n{]}_{-{\phi }^{-2}}!.$

Sequences

OEIS: A003266 Product of first n nonzero Fibonacci numbers F(1), ..., F(n).

OEIS: A059709 and OEIS: A053408 for n such that n!_F − 1 and n!_F + 1 are primes, respectively.