There's a Gamma in my Pi!

Calculus Level 4

n = 1 Γ ( n ) Γ ( n + 1 ) Γ ( n + 2 ) Γ ( n + 3 ) \sum_{n=1}^{\infty}\;\frac{\Gamma(n)\;\Gamma(n+1)}{\Gamma(n+2)\;\Gamma(n+3)}

Find the value of the expression above such that Γ ( n ) \Gamma{(n)} is the Gamma function.

Give answer to three decimal places.


The answer is 0.105.

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2 solutions

Adrian Castro
Jan 14, 2016

By definition of the Gamma function, n = 1 Γ ( n ) Γ ( n + 1 ) Γ ( n + 2 ) Γ ( n + 3 ) = n = 1 ( n 1 ) ! n ! ( n + 1 ) ! ( n + 2 ) ! = n = 1 1 n ( n + 1 ) 2 ( n + 2 ) . \sum_{n=1}^{\infty}\frac{\Gamma(n)\;\Gamma(n+1)}{\Gamma(n+2)\;\Gamma(n+3)}=\sum_{n=1}^{\infty}\frac{(n-1)!\;n!}{(n+1)!\;(n+2)!}=\sum_{n=1}^{\infty}\frac{1}{n(n+1)^2(n+2)}. Decomposition by partial fractions shows that the series can be further reduced to n = 1 ( 1 / 2 n 1 / 2 n + 2 1 ( n + 1 ) 2 ) = 1 2 n = 1 ( 1 n 1 n + 2 ) ( n = 1 1 n 2 1 ) . \sum_{n=1}^{\infty}\left ( \frac{1/2}{n}-\frac{1/2}{n+2}-\frac{1}{(n+1)^2} \right )=\;\frac{1}{2}\sum_{n=1}^{\infty}\left(\frac{1}{n}-\frac{1}{n+2}\right)-\left(\sum_{n=1}^{\infty}\frac{1}{n^2}-1 \right ). The first series on the RHS telescopes giving us

1 2 ( 3 2 ) ( n = 1 1 n 2 1 ) = 7 4 n = 1 1 n 2 = 7 4 π 2 6 0.105 . \frac{1}{2}\left(\frac{3}{2} \right )-\left(\sum_{n=1}^{\infty}\frac{1}{n^2}-1 \right ) =\;\frac{7}{4}-\sum_{n=1}^{\infty}\frac{1}{n^2}=\boxed{\frac{7}{4}-\frac{\pi^2}{6}\approx 0.105}.

Anand Chitrao
Jan 14, 2016

After simplifying, the nth term turns out to be (1/(n)(n+1)(n+1)(n+2)). Which simplifies to (1/(n)-1/(n+1))(1/(n+1)-1/(n+2)). Which after multiplying and simplifying further, turns out to be 0.5×(1/(n) - 1/(n+2)) - (1/n+1)^2. The first term telescopes to 3/4 whereas the second term becomes (π^2)/6 - 1. Hence, the answer is 3/4 - (π^2)/6 +1 = 0.105. Of course upto 3 decimal places.

Yup that's right!

Adrian Castro - 5 years, 5 months ago

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