( x + x 1 ) 2 + ( x 2 + x 2 1 ) 2 + ⋯ + ( x 2 7 + x 2 7 1 ) 2
If x is a complex number satisfying x 2 + x + 1 = 0 , determine the value of the expression above.
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Hence, a more general result,
∀ n ∈ Z + , k = 1 ∑ n ( x k + x k 1 ) 2 = 4 ⌊ n / 3 ⌋ + ( n − ⌊ n / 3 ⌋ ) = n + 3 ⌊ n / 3 ⌋
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If n is a positive integer, then floor function is not required!!
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Firstly, it's floor function, not ceiling. Secondly, it is not required only if 3 ∣ n , otherwise it's required.
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@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 would do..
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@Rishabh Jain – It is indeed n divided by 3 . I'm taking the floored value of this division because that term is meant to be the number of positive multiples of 3 which are ≤ n . This is obviously given by ⌊ n / 3 ⌋ , i.e., ⌊ 3 n ⌋ .
Wow never thought about cube root of unity
Same method.
Use of Newton's Identities makes this easier even for someone not accostomed to complex roots.
*accustomed
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Clearly roots of x 2 + x + 1 are ω , ω 2 such that ω 3 = 1 ∙ ω = ω 4 = ω 7 = ω 1 0 = . . . ∙ ω 2 = ω 5 = ω 8 = ω 1 1 = . . . ∙ ω 3 = ω 6 = ω 9 = ω 1 2 = . . . . = 1 Using cyclicity of ω as stated above, required sum (which has the same value at ω as well as ω 2 ) simplifies to: 9 ( ( x + x 1 ) 2 + ( x 2 + x 2 1 ) 2 + ( x 3 + x 3 1 ) 2 ) 9 ( ( ω + ω 2 ) 2 + ( ω 2 + ω ) 2 + ( 1 + 1 ) 2 ( ω 1 = ω 2 ) = 9 × ( ( − 1 ) 2 + ( − 1 ) 2 + 4 ) = 5 4