Puzzle time - the Fibonacci numbers

Klaus

Actually, there's a trivial solution, too : The first Fibonacci number is 0. 0 is a multiple of any number.

jon-nyc

Haha. Let’s agree to go for the non-trivial solution.

Klaus

Do you (already) know the solution? Does it involve advanced math? Even the partial statement that your proposition holds for prime numbers isn't exactly trivial to show.

jon-nyc

I know a very elegant solution that requires no advanced math.

Hint?

jon-nyc

Explore the Fibonacci numbers mod n for a few n

Klaus

Well, the "mod n" sequences seem to loop. In particular, they always seem to come back to 0. Which means that those "0" points can be divided by n.

So if "zero(n)" is the index of the first fibonacci number (different from 0) where Fib(zero(n)) mod n = 0, then n divides the zero(n)-th Fibonacci number.

However, it's not so clear why the "mod n" sequences have that looping structure.

jon-nyc

You’re on track, and one small insight from a solution.

Klaus

I think the proof of looping can be seen from the congruence property of modular arithmetic. In particular, "modulo" is a congruence with respect to Fibonacci numbers:

Fib(n) mod m = (Fib(n-1) mod m) + (Fib(n-2) mod m) (for n >= 2)

Since there are only about m^2 possible pairs of numbers in the "mod n" ring, and Fib(n) mod m only depends on the pair of previous numbers (mod m), it follows that the sequence must necessarily loop.

jon-nyc

That’s right. The process is totally reversible so the 0,1,1 must eventually repeat.

Klaus

I did a tiny bit of programming, and I think it's a nice programming exercise to come up with an elegant way to compute the cycle length of "Fib mod n" sequences.

Here's a 4-liner I came up with.

fibsmod n = map ((`mod` n) . fst) $ iterate (\(a,b) -> (b,a+b)) (0,1)
pairIndex a b (x:y:ys) m = if (a == x) && (b == y) then m else pairIndex a b (y:ys) (m+1)
p (x:y:ys) = pairIndex x y ys 2
take 100 $ map (p . fibsmod) [2..]

It computes the first 100 cycle length:

[3,8,6,20,24,16,12,24,60,10,24,28,48,40,24,36,24,18,60,16,30,48,24,100,84,72,48,14,120,30,48,40,36,80,24,76,18,56,60,40,48,88,30,120,48,32,24,112,300,72,84,108,72,20,48,72,42,58,120,60,30,48,96,140,120,136,36,48,240,70,24,148,228,200,18,80,168,78,120,216,120,168,48,180,264,56,60,44,120,112,48,120,96,180,48,196,336,120,300,50]

jon-nyc

What language is that?

Horace

It's a computer language. Computer programmers use computer languages to "talk" to computers and tell them what to do!

jon-nyc

Interesting observation:

Cycle lengths 2^n = 3*2^(n-1) at least within the first 100. Does that continue?

Klaus

@jon-nyc said in Puzzle time - the Fibonacci numbers:

What language is that?

Haskell.

I bet if you do the same thing in your favorite language you need at least twice as much code, and the code will be less extensible. (throws gauntlet)

jon-nyc

I use 8086 assembler for such tasks.

Doctor Phibes

Are you two about to have a 'my dick is smaller' contest?

Klaus

@Doctor-Phibes Did you know that dick sizes have a Fibonacci distribution? It's a distribution with a very long tail...

jon-nyc

@jon-nyc said in Puzzle time - the Fibonacci numbers:

Interesting observation:

Cycle lengths 2^n = 3*2^(n-1) at least within the first 100. Does that continue?

bump for Klaus and his little 4 line program.

Klaus

@jon-nyc said in Puzzle time - the Fibonacci numbers:

@jon-nyc said in Puzzle time - the Fibonacci numbers:

Interesting observation:

Cycle lengths 2^n = 3*2^(n-1) at least within the first 100. Does that continue?

bump for Klaus and his little 4 line program.

Can you explain? You mean that the cycle length of Fib mod (2^n) is the same as Fib mod (3*2^(n-1))? That doesn't seem to be true.

I uploaded the first 10,000 cycle lengths here, if you want to check this yourself.
https://www.heypasteit.com/clip/0IV18W

jon-nyc

No, the cycle length of Fib mod (2^n) = 3*2^(n-1))

Check Fmod2, Fmod4, Fmod8, Fmod16 etc for their cycle lengths. You’ll get 3,6,12,24,48 etc. I’m wondering if it holds. I think it will.

The New Coffee Room

Puzzle time - the Fibonacci numbers