Gamma Derivative Twice!

Calculus Level 4

Γ ( 3 ) = F γ A + π B C + D E γ \large \Gamma ^{ \prime \prime }\left( 3 \right) ={ F\gamma }^{ A }+\frac { { \pi }^{ B } }{ C } +D-E\gamma

The equation above holds true for integers A , B , C , D A,B,C,D and E E . Find A + B + C + D + E + F A+B+C+D+E+F .

Notations :

  • Γ ( ) \Gamma(\cdot) denotes the Gamma function . And Γ \Gamma'' denotes the second derivative of the Gamma function.

  • γ \gamma denote the Euler-Mascheroni constant , γ 0.5772 \gamma \approx 0.5772 .


The answer is 17.

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1 solution

Joel Yip
Apr 3, 2016

Using the digamma function's definition, ψ ( x ) = Γ ( x ) Γ ( x ) Γ ( x ) = ψ ( x ) Γ ( x ) d d x ( Γ ( x ) ) = d d x ( ψ ( x ) Γ ( x ) ) Γ ( x ) = ψ 1 ( x ) Γ ( x ) + ψ ( x ) Γ ( x ) u s i n g p r o d u c t r u l e = ψ 1 ( x ) Γ ( x ) + ψ ( x ) 2 Γ ( x ) \psi \left( x \right) =\frac { \Gamma ^{ \prime }\left( x \right) }{ \Gamma \left( x \right) } \\ \Gamma ^{ \prime }\left( x \right) =\psi \left( x \right) \Gamma \left( x \right) \\ \frac { d }{ dx } \left( \Gamma ^{ \prime }\left( x \right) \right) =\frac { d }{ dx } \left( \psi \left( x \right) \Gamma \left( x \right) \right) \\ \Gamma ^{ \prime \prime }\left( x \right) ={ \psi }_{ 1 }\left( x \right) \Gamma \left( x \right) +\psi \left( x \right) \Gamma ^{ \prime }\left( x \right) \quad using\quad product\quad rule\\ ={ \psi }_{ 1 }\left( x \right) \Gamma \left( x \right) +{ \psi \left( x \right) }^{ 2 }\Gamma \left( x \right)

if x = 3 x=3

ψ 1 ( 3 ) Γ ( 3 ) + ψ ( 3 ) 2 Γ ( 3 ) = ( π 2 6 5 4 ) × 2 + ( 3 2 γ ) 2 × 2 = π 2 3 5 2 + 9 2 6 γ + 2 γ 2 = 2 + 2 γ 2 + π 2 3 6 γ \\ { \psi }_{ 1 }\left( 3 \right) \Gamma \left( 3 \right) +{ \psi \left( 3 \right) }^{ 2 }\Gamma \left( 3 \right) =\left( \frac { { \pi }^{ 2 } }{ 6 } -\frac { 5 }{ 4 } \right) \times 2+{ \left( \frac { 3 }{ 2 } -\gamma \right) }^{ 2 }\times 2\\ =\frac { { \pi }^{ 2 } }{ 3 } -\frac { 5 }{ 2 } +\frac { 9 }{ 2 } -6\gamma +2{ \gamma }^{ 2 }\\ =2+2{ \gamma }^{ 2 }+\frac { { \pi }^{ 2 } }{ 3 } -6\gamma

so 2 + 2 + 3 + 2 + 6 + 2 = 17 2+2+3+2+6+2=17

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