I've stumble across the following exercise in a test. I'm trying to solve it but I need some help.
Let $(a_n)_{n \in \Bbb N}$ be the sequence defined as: $$a_1 = 14$$ $$a_{n+1}=5 a_n - 6$$ Prove that the gcd between two consecutive terms is constant and find its value.
For instance by checking the first terms of the sequence I've come up with 2 as a common factor. I can prove this by induction:
$P(n) = 2 \mid a_n $
$P(1) = 2 \mid 14$
$P(n) \implies P(n+1)$:
$2\mid 5 a_n - 6$
As 2 divides 6 it must be that 2 divides $a_n$ because it does not divide 5, and so it happens to be my hypothesis.
The same way I've proved that 4 doesn't divide $a_n$ so 1 is the highest power of 2 that divides the sequence.
Then I've tried to find the $\gcd$ doing as above for two consecutive terms of the sequence and with a generic $\gcd=d$:
$\gcd(a_n, a_{n+1})=\gcd(a_n, 5 a_n - 6)=d$
$d\mid 5 a_n - 6$
As d divides $a_n$ it must be that it also divides 6, then the possible values are 1,2,3,6.
I can see that the only one that is common to my previous assumptions is 2. But I can't put all the information together. Any suggestions will be helpful, thanks!
If $d\mid a_n$, and $d\mid a_{n+1}=5a_n-6$, then $d\mid 6a_n-6=6(a_n-1)$. Now let $d=ij$, such that $i\mid 6$ and $j\mid a_n-1$. Note that $j\mid d\mid a_n$ and since $j\mid a_n-1$, we know $j=1$. Thus, $d=ij=i$ so that $d\mid 6$.
It is clear that $2\mid a_n$ (easy with induction; $2\mid a_1$, and $2\mid a_n$ means $2\mid 5a_n-6=a_{n+1}$). We also know $a_0\equiv 2\mod 3$. Assume now $a_n\equiv 2\mod 3$; then $5a_n\equiv 1\mod 3$ so that $5a_n-6\equiv a_{n+1}\equiv 1\mod 3$. Now $5a_{n+1}\equiv 2\mod 3$ meaning $5a_{n+1}-6\equiv a_{n+2}\equiv 2\mod 3$, so that $a_n$ is never divisible by $3$.
Thus, the $\gcd$ is constant and equal to $2$.