11 is really lovely, we'll bring it everywhere!

Calculus Level 3

Let the sum of the following infinite sequence be N N . 1 , 1 2 , 1 2 , 1 6 , 1 4 , 1 12 , 1 8 , 1 20 , 1 16 , 1 30 , 1 32 , 1 42 . 1,\dfrac{1}{2},\dfrac{1}{2},\dfrac{1}{6},\dfrac{1}{4},\dfrac{1}{12},\dfrac{1}{8},\dfrac{1}{20},\dfrac{1}{16},\dfrac{1}{30},\dfrac{1}{32},\dfrac{1}{42} \ldots \ . Find the remainder \textbf{remainder} when ( N 2014 + 1 ) \displaystyle \Bigl(N^{2014}+1\Bigr) is divided by 11 \color{#69047E}{\textbf{11}} .


Hint :- Is it really a \textbf{a} sequence?


The answer is 5.

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1 solution

Aditya Raut
Sep 20, 2014

See that the sequence is actually inter-twined mixing of 2 2 sequences 1 , 1 2 , 1 2 , 1 6 , 1 4 , 1 12 , 1 8 , 1 20 , 1 16 , 1 30 , 1 32 , 1 42 . . . . \color{#3D99F6}{1},\color{#624F41}{\dfrac{1}{2}},\color{#3D99F6}{\dfrac{1}{2}},\color{#624F41}{\dfrac{1}{6}},\color{#3D99F6}{\dfrac{1}{4}},\color{#624F41}{\dfrac{1}{12}},\color{#3D99F6}{\dfrac{1}{8}},\color{#624F41}{\dfrac{1}{20}},\color{#3D99F6}{\dfrac{1}{16}},\color{#624F41}{\dfrac{1}{30}},\color{#3D99F6}{\dfrac{1}{32}},\color{#624F41}{\dfrac{1}{42}}....

Out of these, the sequence in Blue \color{#3D99F6}{\textbf{sequence in Blue}} is S = 1 + 1 2 + 1 4 + 1 8 . . . \displaystyle S_\infty= \color{#3D99F6}{1+\dfrac{1}{2}+\dfrac{1}{4}+\dfrac{1}{8} }... which is a Geometric progression (GP) with a = 1 , r = 1 2 a=1,r=\dfrac{1}{2} , so it's infinite sum is

a 1 r = 1 1 1 2 = 1 1 2 = 2 \bullet \quad \dfrac{a}{1-r}\\ = \dfrac{1}{1-\frac{1}{2}}\\ = \dfrac{1}{\frac{1}{2}}\\ =\mathbf{2}


The sequence in Brown \color{#624F41}{\textbf{sequence in Brown}} is 1 1 × 2 + 1 2 × 3 + 1 3 × 4 + 1 4 × 5 . . . . . \color{#624F41}{\dfrac{1}{1\times 2} + \dfrac{1}{2\times 3}+\dfrac{1}{3\times 4}+\dfrac{1}{4\times 5}}.....

This can be written as

2 1 1 × 2 + 3 2 2 × 3 + 4 3 3 × 4 + 5 4 4 × 5 + . . . . \dfrac{2-1}{1\times 2}+\dfrac{3-2}{2\times 3}+\dfrac{4-3}{3\times 4}+\dfrac{5-4}{4\times 5}+....

= ( 1 1 2 ) + ( 1 2 1 3 ) + ( 1 3 1 4 ) + ( 1 4 1 5 ) + . . . . A telescoping series = 1 = \biggl(1-\dfrac{1}{2}\biggr)+\biggl(\dfrac{1}{2}-\dfrac{1}{3}\biggr)+\biggl(\dfrac{1}{3}-\dfrac{1}{4}\biggr)+\biggl(\dfrac{1}{4}-\dfrac{1}{5}\biggr)+....\quad \text{A telescoping series}\\ =\quad \mathbf{1}


This gives N = 3 N=3 .

We want to find 3 2014 + 1 ( m o d 11 ) 3^{2014}+1 \pmod{11}

Observe that

3 5 = 243 1 ( m o d 11 ) 3 5 k 1 ( m o d 11 ) 3 2010 1 ( m o d 11 ) 3 2014 3 4 81 4 ( m o d 11 ) 3^5 = 243\equiv 1 \pmod{11} \implies 3^{5k} \equiv 1 \pmod{11} \\ \therefore 3^{2010} \equiv 1 \pmod{11} \\ \therefore 3^{2014} \equiv 3^{4} \equiv 81 \equiv{4} \pmod{11}

Thus, asked answer will be 3 2014 + 1 4 + 1 5 ( m o d 11 ) 3^{2014}+1\equiv 4+1 \equiv \boxed{5} \pmod{11}

well, we can also do 3 2014 {3}^{2014} by fermat's little theorem! I did the rest exactly the same! Nice!

Kartik Sharma - 6 years, 8 months ago

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Would you please explain how .I know the theorem but how to apply it here?

mihir yadav - 6 years, 8 months ago

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3 10 {3}^{10} = 1 mod 11

3 2010 {3}^{2010} = 1 mod 11

Now, we have to find 3 4 {3}^{4} or 81 mod 11 which is just 4.

Kartik Sharma - 6 years, 8 months ago

They say solving math problems gives one a rush of adrenaline. This one is not for the faint of heart then!! Really nice problem and amazing solution.

B.S.Bharath Sai Guhan - 6 years, 8 months ago

nice problem @Aditya Raut

math man - 6 years, 8 months ago

nice one bro

Ashu Dablo - 6 years, 8 months ago

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When you like a problem/solution/note (not just mine, any other you come across...) , then if you admire it, hit the like, reshare , upvote buttons to show your appreciation. That's a better option for this ! Thank you.

Aditya Raut - 6 years, 8 months ago

@Aditya Raut...NICE EXPLAINATION!! But, in the telescoping series, how are you left with just the '1'...why not the last term as well? Is it because we don't know what is it?

A Former Brilliant Member - 6 years, 8 months ago

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It is because the last term is 1/infinity=0

zayan alam - 6 years, 8 months ago

Infinite series, limits, divergence!!

Krishna Ar - 6 years, 8 months ago

Zayam Alam said it right, see that as the series goes further, the denominator goes on increasing, the overall number increasing. If you go on continuing, as he said, at very very large denominators, the terms will be tending to 0 0 and because it's till infinity, 1 = 0 \dfrac{1}{\infty}=0 , makes the sum 1 1 .

Aditya Raut - 6 years, 8 months ago

Aargh!! Stupid dumb me!! @Aditya Raut -I got all of this, wasted two of my tries in finding the remainder part and gave up! Should've computer-bashed it!! :(

Krishna Ar - 6 years, 8 months ago

Very nice problem. The series part has a couple of nice parts to go around :)

A Former Brilliant Member - 6 years, 8 months ago

Nice problem! Arranged the 2 infinity sums a little different, nice with the geometric sequences though. Haven't really seen that before!

Rasmus Brammer - 6 years, 6 months ago

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