Is Binet's formula for the Fibonacci numbers exact?

Question

Is Binet's formula for the Fibonacci numbers exact?

$F_n = \frac{(1+\sqrt{5})^n-(1-\sqrt{5})^n}{2^n \sqrt{5}}$

If so, how, given the irrational numbers in it?

Thanks.

Answer 1

As others have noted, the $\sqrt 5$ parts cancel, leaving an integer. We can recover the Fibonacci recurrence formula from Binet as follows:

$$F_n+F_{n-1} = \frac{(1+\sqrt{5})^n-(1-\sqrt{5})^n}{2^n \sqrt{5}}+\frac{(1+\sqrt{5})^{n-1}-(1-\sqrt{5})^{n-1}}{2^{n-1} \sqrt{5}}=$$$$\frac{(1+\sqrt{5})^{n-1}(1+\sqrt 5+2)-(1-\sqrt{5})^{n-1}(1-\sqrt5+2)}{2^n \sqrt{5}}$$

Then we notice that $(1\pm\sqrt5)^2=6\pm2\sqrt 5=2(3\pm\sqrt5)$

And we use this to simplify the final expression to $F_{n+1}$ so that $F_n+F_{n-1} =F_{n+1}$

And the recurrence shows that if two successive $F_r$ are integers, every Fibonacci number from that point on is an integer. Choose $r=0,1$. This is another way of proving that the cancellation happens.

Answer 2

It is exact, all right. When you expand the powers in the numerators the alternating signs mean that all the surviving terms are of the form an integer times $\sqrt5$. Therefore all the $\sqrt5$s cancel.

Try it out with $n=2$ and $n=3$.

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It may be of interest to you to observe that as $|1-\sqrt5|/2\approx0.618$ its powers quickly approach zero. So for sizable $n$, you can drop that term, and just round the dominant term to the nearest integer. For example with $n=8$ you get $F_8=21$ and $(1+\sqrt5)^8/(2^8\sqrt5)\approx21.009$.

Answer 3

For linear reccurent sequence you can find expression depending on the roots of the associated polynom.

$$ F_{n+1} = F_{n} + F_{n-1} $$

is associated to

$$ x^2 = x +1 $$

Wich has two solution, $ \frac{1 - \sqrt{5}}{2} $ and $ \frac{1 + \sqrt{5}}{2} $ (the golden ratio, a more than interesting number)

Simple roots give a general solution of the form:

$$F_{n} = A * (\frac{1 - \sqrt{5}}{2})^{n} + B * (\frac{1 + \sqrt{5}}{2})^n $$

To determine A and B you have to input initial contidions:

$$F_{0} = A + B = 0 $$

So $$ B = - A $$

and

$$F_{1} = -B * (\frac{1 - \sqrt{5}}{2} - \frac{1 + \sqrt{5}}{2}) =1 $$

$$ B = \frac{1}{\sqrt{5}} $$

So

$$F_{n} = \frac{ (1 + \sqrt{5})^{n} - (1 - \sqrt{5})^n}{2^n\sqrt{5}} $$

By Solving $F_0 = i$ and $F_1 = j$ you can find the general expression of Fibonnaci sequence with starting terms i and j.

Answer 4

$${(1+\sqrt5)^n-(1-\sqrt5)^n} \over 2^n \sqrt5$$

Expand the numerator:

$$(1+\sqrt5)^n-(1-\sqrt5)^n=\sum_{k=0}^{n}{\binom{k}{n}\sqrt5^k}-\sum_{k=0}^{n}{\binom{k}{n}(-1)^k\sqrt5^k}$$

In the second sum, all the even powered terms get a positive sign, which becomes a negative sign due to the fact that the second sum is being subtracted. Those cancel out with the first sum and we get:

$$(1+\sqrt5)^n-(1-\sqrt5)^n=2\sum_{0\leq2k\leq n}{\binom{2k+1}{n}\sqrt5^{2k+1}}$$

We're dividing this by $2^n\sqrt5$ so we get:

$$\frac{2}{2^n}\sum_{0\leq2k\leq n}{\binom{2k+1}{n}\sqrt5^{2k}}$$ $$=\frac{1}{2^{n-1}}\sum_{0\leq2k\leq n}{\binom{2k+1}{n}5^k}$$

Though I have to say, I am a bit stumped as to why this should be an integer.