Large Fibonacci Numbers

Find the remainder when

F 2016 + F 2017 F_{2016}+F_{2017}

is divded by 5, where F n F_n denotes the n th n^\text{th} Fibonacci number .


The answer is 4.

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1 solution

Since there is only 5 5 = 25 5 \cdot 5 = 25 possible remainders that a pair of consecutive Fibonacci numbers can have upon division by 5 5 , the sequence of remainders must become periodic with period 25. \le 25.

So we can check the remainders of the Fibonacci numbers upon division by 5, which is as follows;

1 , 1 , 2 , 3 , 0 , 3 , 3 , 1 , 4 , 0 , 4 , 4 , 3 , 2 , 0 , 2 , 2 , 4 , 1 , 0 , 1 , 1 , . . . . 1,1,2,3,0,3,3,1,4,0,4,4,3,2,0,2,2,4,1,0,1,1,....

We find that the period is 20 20 . Hence

F 2016 + F 2017 F 16 + F 17 2 + 2 4 ( m o d 5 ) . F_{2016}+F_{2017} \equiv F_{16}+F_{17} \equiv 2 + 2 \equiv 4 \pmod 5.

Could you prove it to be periodic for all primes??

Mihir Mallick - 3 years, 5 months ago

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