# What are the first 3 digits of the product of the first 1000 fibonacci numbers

What are the first 3 digits of the product of the first 1000 Fibonacci numbers?

Could anyone give me hints on how to start this problem? I haven't done a problem like this before and I am curious on how to approach such a problem.

-
First three digits from left to right? From right to left? – Andrés E. Caicedo Jul 1 '13 at 7:13
From left to right I believe. – Ozera Jul 1 '13 at 7:15
What do you mean, you believe? You are the one asking the question. – Andrés E. Caicedo Jul 1 '13 at 7:16
@Andres: The question What are the first $3$ digits ... numbers. – Brian M. Scott Jul 1 '13 at 7:26
Brian would be right on "Or perhaps just the one transmitting the question" – Ozera Jul 1 '13 at 7:28

Binet's formula gives us a useful approximation ($\tau=(1+\sqrt5)/2$) $$F_n\approx\frac{\tau^n}{\sqrt5}.$$ For example with $n=8$ the r.h.s. is $21.00952$.

We have $F_1F_2\cdots F_7=3120$, so \begin{aligned} \log(\prod_{i=1}^{1000}F_i)&\approx\log(3120)+\sum_{i=8}^{1000}(i\log\tau-\log\sqrt5)\\ &=\log(3120)+500472\log\tau-\frac{993}2\log5\\ &\approx 104248.9178386, \end{aligned} so using the fractional part of that gives $$10^{0.9178386}\approx8.27635.$$ Thus the answer is $827$ or something close to it. I skipped the estimation of the error. Note that the error to Binet's formula alternates and tends to zero, so it is not too difficult to estimate it.

-
The error in $F_8$ is approximately one part in 2000. Some of that will get cancelled by the error in $F_9$, but this gives us a reason to believe that the answer is correct up to 3 significant digits. – Jyrki Lahtonen Jul 1 '13 at 9:29

Assuming we have agreement that the sequence starts with: {1, 1, 2, 3, 5, 8, 13, ...}, then one can just compute it. Here is the product using Mathematica:

Product[Fibonacci[i], {i, 1, 1000}]


827363170408544049180720689718382309620058799258572848699373843 ... etc etc goes on for a few pages.

-

Hint: $F_n$ is very close to $\varphi^n/\sqrt{5}$, where $\varphi = \ldots$. You should be able to use this approximation for $N < n \le 1000$ (for suitable $N$, hopefully not too big), and compute $F_1 \ldots F_N$ exactly. It may be best to work with logarithms rather than with the product directly.

-