No Fibbing 4

G x = n = 1 x F n F n + 1 F n + 2 G_x = \sum_{n=1}^x F_n F_{n+1} F_{n+2}

If G x G_x is defined as above, where F n F_n denotes the n n th Fibonacci number with F 1 = F 2 = 1 F_1 = F_2 = 1 , then

x = 1 G x G x + 2 G x + 1 2 = a + b c \prod_{x=1}^{\infty} \frac {G_x G_{x+2}}{G_{x+1}^2} = \frac {a + \sqrt{b}}{c}

where a a , b b , and c c are positive integers with b b being square-free. What is a + b + c a + b + c ?


The answer is 11.

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1 solution

Mark Hennings
Jul 15, 2019

Since x = 1 X G x G x + 2 G x + 1 2 = x = 1 X G x × x = 3 X + 2 G x x = 2 X + 1 G x 2 = G 1 G 2 ( x = 3 X G x 2 ) G X + 1 G X + 2 G 2 2 ( x = 3 X G x 2 ) G X + 1 2 = G 1 G X + 2 G 2 G X + 1 \prod_{x=1}^X \frac{G_xG_{x+2}}{G_{x+1}^2} \; = \; \frac{\prod_{x=1}^X G_x \times \prod_{x=3}^{X+2}G_x}{\prod_{x=2}^{X+1}G_x^2} \; = \; \frac{G_1G_2\left(\prod_{x=3}^X G_x^2\right)G_{X+1}G_{X+2}}{G_2^2\left(\prod_{x=3}^XG_x^2\right)G_{X+1}^2} \; = \; \frac{G_1G_{X+2}}{G_2G_{X+1}} for any X 1 X \ge 1 we see that x = 1 G x G x + 2 G x + 1 2 = lim X x = 1 X G x G x + 2 G x + 1 2 = 1 4 lim X G X + 2 G X + 1 = 1 4 ( 2 + 5 ) \prod_{x=1}^\infty \frac{G_xG_{x+2}}{G_{x+1}^2} \; = \; \lim_{X \to \infty} \prod_{x=1}^X\frac{G_xG_{x+2}}{G_{x+1}^2} \; = \; \tfrac14\lim_{X \to \infty} \frac{G_{X+2}}{G_{X+1}} \; = \; \tfrac14(2+\sqrt{5}) using problem 2 in this series. Thus the answer is 2 + 5 + 4 = 11 2+5+4=\boxed{11} .

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