Can't L'Hôpital this one

Calculus Level 5

lim x 0 x ! + α x 1 x 2 \large \lim_{x\to 0}\dfrac{x!+\alpha x-1}{x^2}

The given limit has a finite result. Approximate the value of α \alpha .

Bonus: Evaluate the limit.


The answer is 0.57721.

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1 solution

lim x 0 x ! + α x 1 x 2 \lim_{x\to0} \frac{x! + \alpha x -1}{x^{2}} We can rewrite the factorial function as its definition, the Γ ( x ) \Gamma(x) function lim x 0 Γ ( x + 1 ) + α x 1 x 2 \lim_{x\to0} \frac{\Gamma(x+1) + \alpha x -1}{x^{2}} Because the limit is in the form 0 0 \frac{0}{0} , (note that Γ ( 1 ) = 1 \Gamma(1) = 1 ), we can us L'Hôpital's rule lim x 0 Γ ( x + 1 ) + α 2 x \lim_{x\to0} \frac{\Gamma^{\prime}(x+1) + \alpha}{2x} For the limit to give a finite result, it must not diverge. The limit will not diverge if the fraction is in the form 0 0 \frac{0}{0} , (the form \frac{\infty}{\infty} is unattainable in this case). Thus, Γ ( x + 1 ) = α \Gamma^{\prime}(x+1) = -\alpha Evaluating at x = 0 x=0 Γ ( 1 ) = α \Gamma^{\prime}(1) = -\alpha Evoking the definition of the Γ ( x ) \Gamma(x) function leads to Γ ( x ) = 0 e t t x 1 d t \Gamma(x) = \int_{0} ^{\infty} e^{-t}\;t^{x-1} \;\mathrm{d}t Taking the derivative on both sides with respect to x x , (I am ignoring the Leibniz Rule of differentiation under the integral sign. For more info, look at notable resources below) Γ ( x ) = 0 e t t x 1 ln ( t ) d t \Gamma^{\prime}(x) = \int_{0} ^{\infty} e^{-t}\;t^{x-1}\;\ln(t) \;\mathrm{d}t Evaluating at x = 1 x=1 Γ ( 1 ) = 0 e t ln ( t ) d t \Gamma^{\prime}(1) = \int_{0} ^{\infty} e^{-t}\;\ln(t) \;\mathrm{d}t This is one of the integral definitions of the Euler-Mascheroni constant, thus Γ ( 1 ) = γ = α \Gamma^{\prime}(1) = -\gamma = -\alpha α = γ = 0.57721 \alpha = \gamma = \boxed{0.57721} In order to evaluate the limit, which is now lim x 0 Γ ( x + 1 ) + γ 2 x \lim_{x\to0} \frac{\Gamma^{\prime}(x+1) + \gamma}{2x} we will evoke L'Hôpital's rule once more lim x 0 Γ ( x + 1 ) 2 \lim_{x\to0} \frac{\Gamma^{\prime\prime}(x+1)}{2} Γ ( 1 ) 2 \frac{\Gamma^{\prime\prime}(1)}{2} Using the Γ ( x ) \Gamma^{\prime}(x) from earlier, which was Γ ( x ) = 0 e t t x 1 ln ( t ) d t \Gamma^{\prime}(x) = \int_{0} ^{\infty} e^{-t}\;t^{x-1}\;\ln(t) \;\mathrm{d}t Taking the derivative once more on both sides with respect to x x Γ ( x ) = 0 e t t x 1 ln 2 ( t ) d t \Gamma^{\prime\prime}(x) = \int_{0} ^{\infty} e^{-t}\;t^{x-1}\;\ln^{2}(t) \;\mathrm{d}t Setting x = 1 x=1 Γ ( 1 ) = 0 e t ln 2 ( t ) d t \Gamma^{\prime\prime}(1)= \int_{0} ^{\infty} e^{-t}\;\ln^{2}(t) \;\mathrm{d}t Γ ( 1 ) = γ 2 + π 2 6 \Gamma^{\prime\prime}(1) = \gamma^{2} + \frac{\pi^{2}}{6}

Thus our limit is then equal to γ 2 + π 2 6 2 0.989 \frac{\gamma^{2} + \frac{\pi^{2}}{6}}{2} \approx 0.989

Notable Resources:

In depth analysis of the derivative of the Gamma Function

Gamma Function

Euler-Mascheroni constant

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