Psi Psi

Calculus Level 3

Evaluate : ψ ( 3 2 ) \psi \left ( \frac{3}{2} \right )

The format of the solution is as give below :

A ln ( B ) γ A - \ln (B) -\gamma

Submit your answer as A + B A+B if A A and B B are positive integers.

Clarifications :

γ \gamma denotes Euler–Mascheroni constant

ψ \psi denotes Digamma function


The answer is 6.

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1 solution

ψ ( 1 + z ) = γ + 0 1 1 x z 1 x d x \psi(1+z) = -\gamma + \displaystyle \int_{0}^{1} \dfrac{1-x^{z}}{1-x}dx
Putting z = 1 2 z = \dfrac{1}{2}
ψ ( 3 2 ) = γ + 0 1 1 x 1 x d x = γ + 0 1 1 1 + x d x \psi\left(\dfrac{3}{2}\right) = -\gamma + \displaystyle \int_{0}^{1} \dfrac{1-\sqrt{x}}{1-x}dx = -\gamma + \displaystyle \int_{0}^{1} \dfrac{1}{1+\sqrt{x}}dx
Put x = t 2 d x = 2 t d t x = t^{2} \longrightarrow dx = 2tdt
ψ ( 3 2 ) = γ + 2 0 1 t 1 + t d t \therefore \psi\left(\dfrac{3}{2}\right) = -\gamma + \displaystyle 2\int_{0}^{1} \dfrac{t}{1+t}dt
ψ ( 3 2 ) = γ + 2 0 1 1 d t 2 0 1 1 1 + t d t \psi\left(\dfrac{3}{2}\right) = -\gamma + \displaystyle 2\int_{0}^{1} 1dt - \displaystyle 2\int_{0}^{1}\dfrac{1}{1+t}dt
ψ ( 3 2 ) = γ + [ 2 t ] 0 1 [ 2 ln ( 1 + t ) ] 0 1 \psi\left(\dfrac{3}{2}\right) = -\gamma + \left[2t\right]_{0}^{1} - \left[2\ln\left(1+t\right)\right]_{0}^{1}
ψ ( 3 2 ) = γ + 2 2 ln ( 2 ) \psi\left(\dfrac{3}{2}\right) = -\gamma + 2 - 2\ln(2)


ψ ( 3 2 ) = γ + 2 ln ( 4 ) \psi\left(\dfrac{3}{2}\right) = -\gamma + 2 - \ln(4)

A + B = 2 + 4 = 6 A + B = 2+4 = 6

Moderator note:

Good clear solution.

Great ! Thanks for writing wonderful solution. ...

A Former Brilliant Member - 5 years, 3 months ago

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