n → ∞ lim k = 0 ∑ ⌊ n / 3 ⌋ ( 3 k n ) k = 0 ∑ n ( k n ) = ?
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n does not need to be an odd multiple of 3 , or even a multiple of 3 . Just replace n / 3 by ⌊ n / 3 ⌋ at the top of the sum in the denominator, and you are good to go, with 3 k = 0 ∑ ⌊ n / 3 ⌋ ( 3 k n ) = { 2 n + 2 ( − 1 ) n 2 n − ( − 1 ) n n ≡ 0 ( m o d 3 ) n ≡ 0 ( m o d 3 )
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Yeah, I only realized this after I made the question and took the limit.
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k = 0 ∑ n ( k n ) = ( 1 + 1 ) n = 2 n k = 0 ∑ ⌊ n / 3 ⌋ ( 3 k n ) = 3 ( 1 + 1 ) n + ( − ω ) n + ( − ω 2 ) n = 3 2 n − 1 − 1 = 3 2 ( 2 n − 1 − 1 ) ⟹ 2 n ÷ 3 2 ( 2 n − 1 − 1 ) = 2 ( 2 n − 1 − 1 ) 3 × 2 n = 2 n − 1 − 1 3 ⋅ 2 n − 1 ∴ n → ∞ lim k = 0 ∑ ⌊ n / 3 ⌋ ( 3 k n ) k = 0 ∑ n ( k n ) = n → ∞ lim 2 n − 1 − 1 3 × 2 n − 1 = 3
Note: Here, ω is a third root of unity.