I'm a famous constant

Calculus Level 4

γ n = H n log n , n 1 , x R \gamma_n = H_n - \log n , n\geq 1 , x\in\mathbb R Find Ω ( x ) = lim n ( 1 n i = 1 n γ i sin 2 x γ n + 1 i cos 2 x ) \Omega(x) = \lim_{n\to \infty}\left(\dfrac{1}{n}\sum_{i=1}^n \gamma_i^{\sin^2 x } \cdot \gamma_{n+1-i}^{\cos^2 x}\right)

Find the value log 10 ( Ω ( x ) ) \log_{10}(\Omega(x)) .

This is the proposal of Professor Daniel Sitaru and the problem original source is Romanian Mathematical Mazagine .


The answer is -0.238661.

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1 solution

Naren Bhandari
Jul 8, 2019

This is a very nice analysis problem as it's been also solved by Sir Ali Jaffal from Lebanon in complete analysis way.

Here is my try for solution

For convenience we write sin 2 x = S ( x ) \sin^2x = S(x) and cos 2 x = C ( x ) \cos^2 x = C(x) then we observe that ψ ( n ) = i = 1 n γ i S ( x ) γ n + 1 i C ( x ) = i = 1 n k = n + 1 i n γ i S ( x ) γ k C ( x ) = i = 1 n γ i S ( x ) k = 1 n γ k C ( x ) = m = 1 n γ m S ( x ) + C ( x ) = m = 1 n γ m ( for i = k = m ) \small{\psi(n)= \sum_{i=1}^{n } \gamma_i^{S(x)} \cdot \gamma_{n+1-i}^{C(x)} =\sum_{i=1}^{n}\sum_{k=n+1-i}^n\gamma_i^{S(x)} \gamma_k^{C(x)}=\sum_{i=1}^n\gamma_i^{S(x)}\sum_{k=1}^{n} \gamma_k^{C(x)}\\ =\sum_{m=1}^n \gamma_m^{S(x)+C(x)} =\sum_{m=1}^{n } \gamma_{m} \ \ \ (\text{for } i=k=m )} since S ( x ) + C ( x ) = 1 S(x)+C(x)=1 Now lim n ψ ( n ) n = lim n 1 n m = 1 n γ m = lim n 1 n m = 1 n ( H m log m ) = lim n 1 n ( m = 1 n j = 1 m 1 j log ( m = 1 n m ) ) = lim n 1 n ( m = 1 n n m + 1 m log ( n ! ) ) = lim n ( m = 1 n 1 m n log n n + O log ( n ) n 1 + 1 n m = 1 n 1 n ) Ω ( x ) = lim n ( m = 1 n 1 m log n ) + 0 = γ \lim_{n\to \infty} \dfrac{\psi(n) } {n}=\lim_{n\to \infty} \dfrac{1}{n} \sum_{m=1}^{n} \gamma_m=\lim_{n\to \infty} \dfrac{1}{n} \sum_{m=1}^{n} (H_m -\log m ) \\ = \lim_{n\to \infty} \dfrac{1}{n}\left(\sum_{m=1}^{n} \sum_{j=1}^{m} \dfrac{1}{j} -\log\left(\prod_{m=1}^{n} m\right)\right)=\lim_{n\to \infty} \dfrac{1}{n}\left(\sum_{m=1}^n\dfrac{n-m+1}{m}-\log(n!) \right)\\ = \lim_{n\to \infty}\left(\sum_{m=1}^n\dfrac{1}{m} -\dfrac{n\log n-n +O\log(n) }{n} - 1 +\dfrac{1}{n}\sum_{m=1}^n\dfrac{1}{n} \right) \\ \therefore \Omega(x) =\lim_{n\to \infty}\left(\sum_{m=1}^n\dfrac{1}{m}-\log n\right)+0=\gamma and hence log ( Ω ( x ) ) = log ( γ ) = 0.238661 \log(\Omega(x))=\log(\gamma) = -0.238661\cdots .

You're mixing the notation for logarithms - sometimes " log \log " means "natural logarithm" and sometimes it doesn't.

Chris Lewis - 1 year, 11 months ago

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