Groups of Reciprocal Friends

Algebra Level 4

( x + 1 x ) 2 + ( x 2 + 1 x 2 ) 2 + + ( x 27 + 1 x 27 ) 2 \left( x + \frac{1}{x} \right)^2 + \left( x^2 + \frac{1}{x^2} \right)^2 + \cdots + \left( x^{27} + \frac{1}{x^{27}} \right)^2

If x x is a complex number satisfying x 2 + x + 1 = 0 x^2 + x + 1 = 0 , determine the value of the expression above.


Source: Trinity University


The answer is 54.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

1 solution

Rishabh Jain
Jan 25, 2016

Clearly roots of x 2 + x + 1 are ω , ω 2 such that ω 3 = 1 \small {\color{#3D99F6}{\text{Clearly roots of }x^2+x+1 \text{are }ω,ω^2 \text{such that}~ ω^3=1}} ω = ω 4 = ω 7 = ω 10 = . . . ω 2 = ω 5 = ω 8 = ω 11 = . . . ω 3 = ω 6 = ω 9 = ω 12 = . . . . = 1 \color{forestgreen}{\bullet~ω=ω^4=ω^7=ω^{10}=... \\ {\bullet} ~ω^2=ω^5=ω^8=ω^{11}=... \\ {\bullet}~ ω^3=ω^6=ω^9=ω^{12}=....=1} Using cyclicity of ω as stated above, required sum (which has the same value at ω as well as ω 2 ω^2 ) simplifies to: 9 ( ( x + 1 x ) 2 + ( x 2 + 1 x 2 ) 2 + ( x 3 + 1 x 3 ) 2 ) 9( ({x+\frac { 1 }{ x } ) }^{ 2 }+({ { x }^{ 2 }+\frac { 1 }{ { x }^{ 2 } } ) }^{ 2 }+{ ({ x }^{ 3 }+\frac { 1 }{ { x }^{ 3 } } ) }^{ 2 }) 9 ( ( ω + ω 2 ) 2 + ( ω 2 + ω ) 2 + ( 1 + 1 ) 2 9( (ω+ω^2)^2+(ω^2+ω)^2+(1+1)^2 ( 1 ω = ω 2 ) ~~~~~\small{\color{#D61F06}{(\dfrac{1}{ω}=ω^2)}} = 9 × ( ( 1 ) 2 + ( 1 ) 2 + 4 ) = 54 =9\times((-1)^2+(-1)^2+4)=\Large 54

Hence, a more general result,

n Z + , k = 1 n ( x k + 1 x k ) 2 = 4 n / 3 + ( n n / 3 ) = n + 3 n / 3 \forall~n\in\Bbb{Z^+}~,~\sum_{k=1}^n\left(x^k+\frac{1}{x^k}\right)^2=4\lfloor n/3\rfloor + (n-\lfloor n/3\rfloor)=n+3\lfloor n/3\rfloor

Prasun Biswas - 5 years, 4 months ago

Log in to reply

If n is a positive integer, then floor function is not required!!

Rishabh Jain - 5 years, 4 months ago

Log in to reply

Firstly, it's floor function, not ceiling. Secondly, it is not required only if 3 n 3\mid n , otherwise it's required.

Prasun Biswas - 5 years, 4 months ago

Log in to reply

@Prasun Biswas It's appearing as n divided by 3 that's why I got confused.. :) Yup I had also generalised that by breaking into 3 cases but 3 n 3\mid n would do..

Rishabh Jain - 5 years, 4 months ago

Log in to reply

@Rishabh Jain It is indeed n n divided by 3 3 . I'm taking the floored value of this division because that term is meant to be the number of positive multiples of 3 3 which are n \leq n . This is obviously given by n / 3 \lfloor n/3\rfloor , i.e., n 3 \lfloor\frac n3\rfloor .

Prasun Biswas - 5 years, 4 months ago

Wow never thought about cube root of unity

Department 8 - 5 years, 4 months ago

Log in to reply

Yup.... ω \large ω is amazing. :D

Rishabh Jain - 5 years, 4 months ago

Same method.

Kushagra Sahni - 5 years, 4 months ago

Use of Newton's Identities makes this easier even for someone not accostomed to complex roots.

Aniruddha Bagchi - 4 years, 3 months ago

*accustomed

Aniruddha Bagchi - 4 years, 3 months ago

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...