Evaluate the following limit,
Notations:
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The Weierstrass form of the Gamma function is Γ ( z ) = [ z e γ z k = 1 ∏ ∞ ( 1 + k z ) e − z / k ] − 1 Taking its logarithmic differentation, Γ ( z ) Γ ′ ( z ) = z 1 + γ + n = 1 ∑ ∞ ( 1 + z / n 1 / n − n 1 ) . Thus, the first few terms of the Maclaurin series of Γ ( z ) is Γ ( z ) = z 1 + γ + z ( ⋯ ) And because 1 + x 1 − 1 = − k = 2 ∑ ∞ ( − 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 where ζ ( ⋅ ) denotes the Riemann-Zeta function . Thus, the first few terms of the Maclaurin series of Ψ ( 1 − z ) is − γ ( z ) − ζ ( 2 ) z + ζ ( 3 ) z 2 + z ( ⋯ ) Putting it all together, the limit in question is x → 0 lim Γ ( x ) ( γ + ψ ( 1 − x ) ) = z → 0 lim [ z 1 + γ + z ( ⋯ ) ] ⋅ [ − ζ ( 2 ) z + ζ ( 3 ) z 2 + z ( ⋯ ) ] = − ζ ( 2 ) = − 6 π 2 ≈ − 1 . 6 4 4 9 3 .