Fib

Let f n f_n be the n n th Fibonacci number such that f 1 = f 2 = 1 f_1=f_2=1 and f k = f k 1 + f k 2 f_k=f_{k-1}+f_{k-2} for k 3 k \ge 3 . Is it possible that

f x + f x + 1 + f x + 2 + f x + 3 + f x + 4 + f x + 5 + f x + 6 + f x + 7 = f y f_x+f_{x+1}+f_{x+2}+f_{x+3}+f_{x+4}+f_{x+5}+f_{x+6}+f_{x+7}=f_{y}

where x x and y y are positive integers?

Yes, it's possible. No, it's not possible.

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1 solution

Áron Bán-Szabó
Aug 23, 2017

Note that f x + 2 = f x + f x + 1 f x + 3 = f x + 2 + f x + 1 = f x + 2 f x + 1 f x + 4 = f x + 3 + f x + 2 = 2 f x + 3 f x + 1 f x + 5 = f x + 4 + f x + 3 = 3 f x + 5 f x + 1 f x + 6 = f x + 5 + f x + 4 = 5 f x + 8 f x + 1 f x + 7 = f x + 6 + f x + 5 = 8 f x + 13 f x + 1 \begin{aligned} f_{x+2} & = f_{x}+f_{x+1} \\ \ \\ f_{x+3} & = f_{x+2}+f_{x+1} \\ & = f_x+2f_{x+1} \\ \ \\ f_{x+4} & = f_{x+3}+f_{x+2} \\ & = 2f_x+3f_{x+1} \\ \ \\ f_{x+5} & = f_{x+4}+f_{x+3} \\ & = 3f_x+5f_{x+1} \\ \ \\ f_{x+6} & = f_{x+5}+f_{x+4} \\ & = 5f_x+8f_{x+1} \\ \ \\ f_{x+7} & = f_{x+6}+f_{x+5} \\ & = 8f_x+13f_{x+1} \end{aligned} The sum of the eight numbers is 21 f n + 33 f x + 1 21f_n+33f_{x+1} . Since f x + 8 = 13 f x + 21 f x + 1 f_{x+8}=13f_x+21f_{x+1} and f x + 9 = 21 f x + 34 f x + 1 f_{x+9}=21f_x+34f_{x+1} , f x + 8 < 21 f x + 33 f x + 1 < f x + 9 f_{x+8} \ < \ 21f_x+33f_{x+1} \ < f_{x+9} So f x + f x + 1 + f x + 2 + f x + 3 + f x + 4 + f x + 5 + f x + 6 + f x + 7 f_x+f_{x+1}+f_{x+2}+f_{x+3}+f_{x+4}+f_{x+5}+f_{x+6}+f_{x+7} is between two consecutive fibonacci number.

Therefore it is not possible \boxed{\text{it is not possible}} .

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