Bash Me Not

Let g 0 , g 1 , g 2 , . . . , g n g_{0},g_{1},g_{2},...,g_{n} be a sequence satisfying the Fibonacci recurrence relation such that g n = g n 1 + g n 2 g_{n}=g_{n-1}+g_{n-2} g 0 = 2 , g 1 = 1 g_{0}=2, g_{1}=-1 where 2 n 2 \leq n Find g 496 g_{496} and enter the last two digits. You may use a computational engine if and only if you found the general formula. Before it was the first perfect number now it's the third!


The answer is 13.

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1 solution

Since it follows the Fibonacci recurrence relation, then the general formula would be in the form of g n = a ( 1 + 5 2 ) n + b ( 1 5 2 ) n g_{n}=a(\frac{1+\sqrt{5}}{2})^n + b(\frac{1-\sqrt{5}}{2})^n Now the first two terms given represent: a + b = 2 a+b=2 a ( 1 + 5 2 ) + b ( 1 5 2 ) = 1 a(\frac{1+\sqrt{5}}{2})+b(\frac{1-\sqrt{5}}{2})=-1 Solving this system, we get: a = ( 5 2 5 ) a n d b = ( 5 + 2 5 ) a=(\frac{\sqrt{5}-2}{\sqrt{5}}) ~and ~b=(\frac{\sqrt{5}+2}{\sqrt{5}}) Thus, the formula for g n g_{n} is g n = ( 5 2 5 ) ( 1 + 5 2 ) n + ( 5 + 2 5 ) ( 1 5 2 ) n g_{n}=(\frac{\sqrt{5}-2}{\sqrt{5}})(\frac{1+\sqrt{5}}{2})^n + (\frac{\sqrt{5}+2}{\sqrt{5}})(\frac{1-\sqrt{5}}{2})^n Using Wolfram, we get the last two digits 13 \implies \boxed{13}

also note that g 496 = f 494 g_{496} = f_{494} where f 494 f_{494} denotes the 494 t h 494th Fibonacci term

Marc Vince Casimiro - 6 years, 6 months ago

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Actually, g 496 = F 493 g_{496} = F_{493} .

Jake Lai - 6 years, 6 months ago

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Ah, yes, my mistake, thanks for pointing that out!

Marc Vince Casimiro - 6 years, 6 months ago

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