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Calculus Level pending

Evaluate the following limit,

lim x 0 Γ ( x ) ( γ + ψ ( 1 x ) ) \lim_{x\to 0}\Gamma(x)(\gamma+\psi(1-x))

Notations:


The answer is -1.64493.

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1 solution

Pi Han Goh
Dec 9, 2020

The Weierstrass form of the Gamma function is Γ ( z ) = [ z e γ z k = 1 ( 1 + z k ) e z / k ] 1 \Gamma(z) = \left[z e^{\gamma z} \prod_{k=1}^\infty \left(1 + \frac zk\right) e^{-z/k} \right]^{-1} Taking its logarithmic differentation, Γ ( z ) Γ ( z ) = 1 z + γ + n = 1 ( 1 / n 1 + z / n 1 n ) . \dfrac{\Gamma'(z) }{\Gamma(z)} = \frac 1z + \gamma + \sum_{n=1}^\infty \left( \frac{1/n}{1 + z/n} - \frac1n \right). Thus, the first few terms of the Maclaurin series of Γ ( z ) \Gamma(z) is Γ ( z ) = 1 z + γ + z ( ) \Gamma(z) = \frac 1z + \gamma + z( \cdots) And because 1 1 + x 1 = k = 2 ( 1 ) k x k 1 \frac1{1+x} - 1 = -\sum_{k=2}^\infty (-1)^k x^{k-1} The following digamma series is a direct result, Ψ ( 1 + x ) = γ + k = 2 ( 1 ) k ζ ( k ) x k 1 , x < 1 \Psi(1+x) = -\gamma + \sum_{k=2}^\infty (-1)^k \zeta(k) x^{k-1} ,\quad |x| < 1 where ζ ( ) \zeta(\cdot) denotes the Riemann-Zeta function . Thus, the first few terms of the Maclaurin series of Ψ ( 1 z ) \Psi(1-z) is γ ( z ) ζ ( 2 ) z + ζ ( 3 ) z 2 + z ( ) -\gamma(z) - \zeta(2) z + \zeta(3) z^2 + z(\cdots) Putting it all together, the limit in question is lim x 0 Γ ( x ) ( γ + ψ ( 1 x ) ) = lim z 0 [ 1 z + γ + z ( ) ] [ ζ ( 2 ) z + ζ ( 3 ) z 2 + z ( ) ] = ζ ( 2 ) = π 2 6 1.64493. \lim_{x\to 0}\Gamma(x)(\gamma+\psi(1-x)) = \lim_{z\to 0} \left [ \frac 1z + \gamma + z( \cdots) \right ] \cdot \left [ - \zeta(2) z + \zeta(3) z^2 + z(\cdots) \right ] = -\zeta(2) = -\frac{\pi^2}6 \approx -1.64493.

Have to admit,a really NEAT solution,and on top of that shorter compared to how I solved it.

Yoihenba Laishram - 6 months ago

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