T ( a ) = ⋯ a F 3 a F 2 a F 1 , a > 0
where F n denotes the n th Fibonacci number .
Then T ( 4 ) = α γ β ,where α is a prime and β and γ are co prime positive integers. Then:
α + β + γ = ?
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To be completely rigorous, one needs to explain why P converges ;) (Inspired by @Abhishek Sinha )
. Did the same solution again. I think you'll get bored seeing this comment
Exactly the same way
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Cool... BTW Someone is down voting all our comments secretly.. :-)
The generating function of F n is G ( x ) = ∑ n = 1 ∞ F n x n = 1 − x − x 2 x , and ∑ n = 1 ∞ ( − 1 ) n + 1 2 n F n = − ∑ n = 1 ∞ F n ( − 2 1 ) n = − G ( − 2 1 ) = 5 2 . Thus T ( 4 ) = 4 2 / 5 = 2 4 / 5 . The answer is 2 + 4 + 5 = 1 1 .
To be completely rigorous, one needs to state under what condition the generating function is valid. In other words, what is the region of convergence of G ( x ) ? Does − 1 / 2 fall into it ?
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Sure! The roots of x 2 + x − 1 = 0 are − ϕ , ϕ − 1 , so that the radius of convergence is ϕ − 1 > 2 1 .
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T ( a ) = a 2 F 1 − 2 2 F 2 + 2 3 F 3 + ⋯ P
P = − 2 P = 2 1 − 2 2 1 + 2 3 2 − 2 4 3 + ⋯ − 2 2 1 + 2 3 1 − 2 4 2 + ⋯
Subtracting and multiplying by 2 we get:
3 P = 1 + 2 P 2 2 1 − 2 3 1 + 2 4 2 + ⋯ ⟹ P = 5 2
⟹ T ( a ) = a 5 2 T ( 4 ) = 4 5 2 = 2 5 4
∴ 2 + 4 + 5 = 1 1