Combinatorial limit

Calculus Level 4

lim n k = 0 n ( n k ) k = 0 n / 3 ( n 3 k ) = ? \Large {\lim_{n \to \infty}}{\dfrac{\displaystyle{\large\displaystyle \sum^{n}_{k=0}\dbinom{n}{k}}}{\large\displaystyle \sum^{\lfloor n/3\rfloor}_{k=0}\dbinom{n}{3k}}} = ?


The answer is 3.

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1 solution

Akeel Howell
Mar 5, 2017

k = 0 n ( n k ) = ( 1 + 1 ) n = 2 n \displaystyle{\large\displaystyle \sum^{n}_{k=0}\dbinom{n}{k} = (1+1)^n = 2^n} k = 0 n / 3 ( n 3 k ) = ( 1 + 1 ) n + ( ω ) n + ( ω 2 ) n 3 = 2 n 1 1 3 = 2 ( 2 n 1 1 ) 3 \large\displaystyle{\sum^{\lfloor n/3 \rfloor}_{k=0}\dbinom{n}{3k} = \dfrac{(1+1)^n+(-\omega)^n+(-\omega^2)^n}{3} = \dfrac{2^n-1-1}{3} = \dfrac{2(2^{n-1}-1)}{3}} 2 n ÷ 2 ( 2 n 1 1 ) 3 = 3 × 2 n 2 ( 2 n 1 1 ) = 3 2 n 1 2 n 1 1 lim n k = 0 n ( n k ) k = 0 n / 3 ( n 3 k ) = lim n 3 × 2 n 1 2 n 1 1 = 3 \implies 2^n \div \dfrac{2(2^{n-1}-1)}{3} = \dfrac{3 \times 2^n}{2(2^{n-1}-1)} = \dfrac{3 \cdot 2^{n-1}}{2^{n-1}-1} \\ \therefore \lim_{n \to \infty}{\dfrac{\displaystyle{\large\displaystyle \sum^{n}_{k=0}\dbinom{n}{k}}}{\large\displaystyle \sum^{\lfloor n/3 \rfloor}_{k=0}\dbinom{n}{3k}} = \lim_{n \to \infty}{\dfrac{3 \times 2^{n-1}}{2^{n-1}-1}} = \boxed{3}}


Note: Here, ω \large\omega is a third root of unity.

n n does not need to be an odd multiple of 3 3 , or even a multiple of 3 3 . Just replace n / 3 n/3 by n / 3 \lfloor n/3\rfloor at the top of the sum in the denominator, and you are good to go, with 3 k = 0 n / 3 ( n 3 k ) = { 2 n + 2 ( 1 ) n n 0 ( m o d 3 ) 2 n ( 1 ) n n ≢ 0 ( m o d 3 ) 3\sum_{k=0}^{\lfloor n/3 \rfloor} \binom{n}{3k} \; = \; \left\{ \begin{array}{lll} 2^n + 2(-1)^n & \hspace{1cm} & n \equiv 0 \pmod{3} \\ 2^n - (-1)^n & & n \not\equiv 0 \pmod{3} \end{array} \right.

Mark Hennings - 4 years, 3 months ago

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Yeah, I only realized this after I made the question and took the limit.

Akeel Howell - 4 years, 3 months ago

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