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Algebra Difficulty 8.0 Shortlist Find the answer

Let F(0)=0F(0)=0, F(1)=32F(1)=\frac32, and F(n)=52F(n1)F(n2)F(n)=\frac{5}{2}F(n-1)-F(n-2)
for n2n\ge2.

Determine whether or not $n=01F(2n)$\$\displaystyle{\sum_{n=0}^{\infty}\, \frac{1}{F(2^n)}}\$ is a rational number.

(

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

To determine whether the series n=01F(2n)\sum_{n=0}^{\infty} \frac{1}{F(2^n)} is a rational number, we first need to analyze the behavior and values of the function F(n) F(n) , which is defined recursively.

The recurrence relation given is:

F(n)=52F(n1)F(n2) F(n) = \frac{5}{2}F(n-1) - F(n-2)

with initial conditions:

F(0)=0,F(1)=32. F(0) = 0, \quad F(1) = \frac{3}{2}.

Let's calculate the first few terms of the sequence F(n) F(n) to identify a pattern or closed form expression:

- For n=2 n = 2 :
F(2)=52F(1)F(0)=52×320=154 F(2) = \frac{5}{2}F(1) - F(0) = \frac{5}{2} \times \frac{3}{2} - 0 = \frac{15}{4}

- For n=3 n = 3 :
F(3)=52F(2)F(1)=52×15432=758128=638 F(3) = \frac{5}{2}F(2) - F(1) = \frac{5}{2} \times \frac{15}{4} - \frac{3}{2} = \frac{75}{8} - \frac{12}{8} = \frac{63}{8}

From these calculations, we see a pattern emerging that involves powers of 32\frac{3}{2}. We hypothesize that the solution might have the form:

F(n)=k(32)n F(n) = k \left( \frac{3}{2} \right)^n

Applying this hypothesis:

- Substitute F(n)=k(32)n F(n) = k \left( \frac{3}{2} \right)^n into the recurrence relation:

k(32)n=52k(32)n1k(32)n2 k \left( \frac{3}{2} \right)^n = \frac{5}{2} \cdot k\left( \frac{3}{2} \right)^{n-1} - k\left( \frac{3}{2} \right)^{n-2}

- Simplifying, we get:

(32)2=52×321 \left( \frac{3}{2} \right)^2 = \frac{5}{2} \times \frac{3}{2} - 1

- Solving gives:

94=15411=14 \frac{9}{4} = \frac{15}{4} - 1 \quad \Rightarrow \quad 1 = \frac{1}{4}

This confirms that F(n)=(32)n F(n) = \left(\frac{3}{2}\right)^n is a consistent solution up to multiplicative constant.

By the nature of geometric type sequences, F(n) F(n) simplifies down to evaluate individual terms. In the geometric progression, terms are obtained via powers, indicating a rational relationship as far as calculations hold rational results.

Thus we check the infinite series directly:

n=01F(2n)=n=01(32)2n \sum_{n=0}^{\infty} \frac{1}{F(2^n)} = \sum_{n=0}^{\infty} \frac{1}{\left(\frac{3}{2}\right)^{2^n}}

This series converges since its terms decrease towards zero, and the sum itself is a sum of rational numbers (as each term is a rational number).

Consequently, this summation of such numbers is a rational number:

rational \boxed{\text{rational}}

Therefore, the infinite sum n=01F(2n)\sum_{n=0}^{\infty} \frac{1}{F(2^n)} is indeed a rational number.

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Source: Omni-MATH, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.