2 2 n = 0 ∑ ∞ 4 n F n Let F n denote the n th Fibonacci number , where F 0 = 0 , F 1 = 1 and F n = F n − 1 + F n − 2 for n = 2 , 3 , 4 , … .
Compute the expression above.
This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try
refreshing the page, (b) enabling javascript if it is disabled on your browser and,
finally, (c)
loading the
non-javascript version of this page
. We're sorry about the hassle.
You fail to show that the series converges.
Log in to reply
We can simply prove it by ratio test. First we find n → ∞ lim F n F n + 1 .
We can find it by using Binet's formula:
n → ∞ lim F n F n + 1 = n → ∞ lim ϕ n − ( ϕ − 1 ) n ϕ n + 1 − ( ϕ − 1 ) n + 1 = ϕ 1 < 1
Now coming back to the series, we'll do the ratio test. n → ∞ lim F n F n + 1 . 4 1 = ϕ 1 . 4 1 < 1
Hence, the series converges.
Log in to reply
Exactly! Or we can use the generating function, as I did here , making it a one-line solution.
Problem Loading...
Note Loading...
Set Loading...
Let S denote n = 0 ∑ ∞ 4 n F n .Then we have S ⟹ 4 S ⟹ 4 3 S ⟹ 4 3 S ⟹ S ∴ 2 2 S = n = 0 ∑ ∞ 4 n F n = n = 1 ∑ ∞ 4 n F n [ ∵ F 0 = 0 ] = 4 1 + 4 2 1 + 4 3 2 + 4 4 3 + 4 5 5 + 4 6 8 + … = 4 2 1 + 4 3 1 + 4 4 2 + 4 5 3 + 4 6 5 + 4 7 8 + … = 4 1 + 4 3 1 + 4 4 1 + 4 5 2 + 4 6 3 + 4 7 5 + … [ Subtracting ( 2 ) from ( 1 ) ] = 4 1 + 1 6 S = 1 1 4 = 8 … ( 1 ) … ( 2 )