One of the open questions about the Fibonacci numbers is if there are infinite prime numbers inside the Fibonacci sequence. I wonder if a good approach would be trying to know first if there are infinite odd Fibonacci numbers that are Fibonacci pseudoprimes (the condition is not so strict).
There is a very well known Fibonacci (Lucas) primality test, to verify if a number $n$ is a prime number (but there is also a infinite set of Fibonacci pseudoprimes) that can be applied over the Fibonacci numbers as well: the nth Fibonacci number $F(n)$ is at least a Fibonacci pseudoprime if and only if its associated Fibonacci number, which is the $F(n)^{th}$ Fibonacci number, $F(F(n))$ verifies the Fibonacci primality test condition.
Does somebody knows if that approach has been studied?
I am currently reading "Fibonacci Numbers" of Nikolai Nikolaevich Vorob'ev (recommended!) and this topic is explained briefly as an open issue but no clues about the current status. I have reviewed other questions in the forum and I did not find any reference to the Fibonacci pseudoprimes idea I am writing here. Thanks in advance for any comments, references or help.
UPDATE 2015/04/07
So far I have tried to advance and learn more as follows:
(a) It is already proved that $F(p)$ could be (pseudo)prime if and only if $p$ is prime.
(b) The Fibonacci (pseudo)primality rules for $p$, $F(p)$ are:
- If $p \equiv 1,4\ mod\ 5$, then $F(p) \equiv 1\ mod\ p$, and $p$ is at least Fibonacci pseudoprime.
- If $p \equiv 2,3\ mod\ 5$, then $F(p) \equiv p-1\ mod\ p$, and $p$ is at least Fibonacci pseudoprime.
(c) This same rule applied to F(F(p)) looks like this:
- If $F(p) \equiv 1,4\ mod\ 5$, then $F(F(p)) \equiv 1\ mod\ F(p)$, and $F(p)$ is at least Fibonacci pseudoprime.
- If $F(p) \equiv 2,3\ mod\ 5$, then $F(F(p)) \equiv F(p)-1\ mod\ F(p)$, and $F(p)$ is at least Fibonacci pseudoprime.
(d) By reviewing directly the definitions of modularity, it is possible to verify the primality of $F(F(p))$ directly in terms of $p$ as follows:
$F(p) = p*k_1+n,\ k_1 \in \Bbb N, k_1 \gt 0$
$F(F(p)) = F(p)*k_2+n,\ k_2 \in \Bbb N, k_2 \gt 0$
where $n$ can be only $1$ or $p-1$ if the (pseudo)primality is true.
thus,
$F(F(p)) = (p*k_1+n)*k_2+n = p*k_1*k_2+n*k_2+n = p*k_3+n*(k_2+1)$
$k_1*k_2=k_3 \in \Bbb N, k_3 \gt 0$
finally $F(F(p))$ would be at least a Fibonacci pseudoprime if and only if:
$F(F(p)) \equiv n*(k_2+1) \mod p$, for $n=1$ or $n=p-1$.
where $k_2 = \lfloor\frac{F(F(p))}{F(p)}\rfloor$
E.g:
$p=11$ and $F(p)=89$. Both are $\equiv 1,4\ mod\ 5$.
$F(p)=89 \equiv 1\ mod\ 11$, so $p=11$ is at least a Fibonacci pseudoprime.
If $p=11$ is at least a Fibonacci pseudoprime then $F(11) = 89$ could also be a Fibonacci pseudoprime.
By direct primality test $F(89)=1779979416004714189 \equiv 1\ mod\ 89$, so $F(p)=89$ is at least a Fibonacci pseudoprime.
So applying (d): $F(11) = 89$ is at least a Fibonacci pseudoprime if and only if:
$\ \ \ $a. $F(F(11)) \equiv n*(k_2+1) \mod 11$, and
$\ \ \ $b. $ n*(k_2+1) \mod 11 = (F(F(p))\ mod\ p)$
In this case $n=1$, so if the following $(a)=(b)$ is true, the pseudoprimality is true:
$(k_2+1)\equiv 1(a)\ mod\ 11$
$F(F(11))\equiv 1(b) \ mod\ 11$
where $k_2 = \lfloor\frac{1779979416004714189}{89}\rfloor=1999768719154092$
Both congruences are the same one, $1$, so in this case $89$ is also at least a pseudoprime.
Next steps: trying to understand the properties (d) of the direct relationship between $p$ and $F(F(p))$.