The sum $\frac{2}{3\cdot 6} +\frac{2\cdot 5}{3\cdot 6\cdot 9} +\ldots +\frac{2\cdot5\cdot \ldots \cdot 2015}{3\cdot 6\cdot 9\cdot \ldots \cdot 2019}$ is written as a decimal number. Find the first digit after the decimal point.
Problem
Source: Saint Petersburg MO 2020 Grade 11 Problem 4
Tags: number theory
11.12.2020 09:02
Who can solve it?
11.12.2020 15:18
I see that this sum has a closed-form Wolframalpha $\frac{1}{3}\sum_{n=0}^\infty \prod_{k=0}^n\frac{3k+2}{3k+6}=\frac{2}{3}$ I record this on The product of fraction with arithmetic sequence
12.12.2020 05:25
The result generated by WolframAlpha shows Gamma function as this series belongs to Hypergeometric function I add details for this method on The product of fraction with arithmetic sequence $\sum_{n=0}^\infty\prod_{k=0}^n\frac{ak+b}{ak+c} =\frac{b}{c}~_2F_1(1+\frac{b}{a},1;1+\frac{c}{a};1) =\frac{b\Gamma(1+\frac{c}{a})\Gamma(\frac{c-b}{a}-1)}{c\Gamma(\frac{c}{a})\Gamma(\frac{c-b}{a})} =\frac{b}{c-a-b}$
12.12.2020 12:45
Is there any idea to start( tried V_p but I couldn't proceed)?
12.12.2020 16:35
Note that If $a_n=\frac{2*5* ...*(3n-1)}{3*6*9*...*(3n+3)}$ and $s_n=a_1+...+a_n$ Then $s_n+(3n+2)a_n=\frac{2}{3}$
13.12.2020 00:08
RagvaloD wrote: Note that If $a_n=\frac{2*5* ...*(3n-1)}{3*6*9*...*(3n+3)}$ and $s_n=a_1+...+a_n$ Then $s_n+(3n+2)a_n=\frac{2}{3}$ Good idea. I modify my proof as $(c-a-b)\sum_{n=0}^N\prod_{k=0}^n\frac{ak+b}{ak+c}+(a(N+1)+b)\prod_{k=0}^N\frac{ak+b}{ak+c}=b$