Daily Challenge #65- A Disguised Sequence

Given u0 = 1, u1 = 2 and the relation 6unun+1-5unun+2+un+1un+2 = 0 calculate un for any integer n >= 0.

Example:
fcn(n) returns un: fcn(17) -> 131072, fcn(21) -> 2097152

Note:
You can look at this as

-Purely algorithmic from the definition of u^n
-the second one - not at all mandatory, but as a compliment - is to get a bit your head around and find which sequence is hidden behind u^n

This challenge comes from g964 at CodeWars, who has licensed redistribution of this challenge under the 2-Clause BSD License!

Want to propose a challenge idea for a future post? Email yo+challenge@dev.to with your suggestions!