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Calculus Level 5

n = 0 2 n + 1 n ! 2 ( 2 n + 1 ) ! ( n + 1 ) 2 = A π G B C ζ ( D ) + π E F ln H \large \sum _{ n=0 }^{ \infty }{ \dfrac { { 2 }^{ n+1 }n!^{ 2 } }{ (2n+1)!(n+1)^{ 2 } } } =A\pi G-\dfrac { B }{ C } \zeta (D)+\dfrac { { \pi }^{ E } }{ F } \ln { H }

The equation above holds true for positive integers A , B , C , D , E , F A,B,C,D,E,F and H H , where B , C B,C are coprime and H H is minimized.

Find A + B + C + D + E + F + H A+B+C+D+E+F+H .

Notations :

  • ! ! denotes the factorial notation. For example, 8 ! = 1 × 2 × 3 × × 8 8! = 1\times2\times3\times\cdots\times8 .
  • G G denotes the Catalan's constant , G = n = 0 ( 1 ) n ( 2 n + 1 ) 2 0.916 \displaystyle G = \sum_{n=0}^\infty \dfrac{ (-1)^n}{(2n+1)^2} \approx 0.916 .
  • ζ ( ) \zeta(\cdot) denotes the Riemann zeta function .


The answer is 56.

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1 solution

Let the summation be denoted by S = n = 0 2 n + 1 n ! 2 ( 2 n + 1 ) ! ( n + 1 ) 2 \displaystyle S = \sum _{ n=0 }^{ \infty }{ \dfrac { { 2 }^{ n+1 }n!^{ 2 } }{ (2n+1)!(n+1)^{ 2 } } }

We rewrite the summation as S = n = 1 2 n + 1 n 3 ( 2 n n ) \displaystyle S = \sum_{n=1}^{\infty} \frac{2^{n+1}}{n^3\binom{2n}{n}}

By Gregory Series , y t a n 1 y = m = 1 ( 1 ) m 1 y 2 m 2 m 1 \displaystyle ytan^{-1}y = \sum_{m=1}^{\infty} \frac{(-1)^{m-1}y^{2m}}{2m-1}

Substitute y = x 1 x 2 \displaystyle y=\frac{x}{\sqrt{1-x^2}} ,

x 1 x 2 sin 1 x = m = 1 ( 1 ) m 1 x 2 m ( 2 m 1 ) ( 1 x 2 ) m \displaystyle \frac{x}{\sqrt{1-x^2}}\sin^{-1}x = \sum_{m=1}^{\infty} \frac{(-1)^{m-1}x^{2m}}{(2m-1)(1-x^2)^m}

= m = 1 ( 1 ) m 1 2 m 1 k = 0 ( m + k 1 k ) x 2 ( k + m ) \displaystyle = \sum_{m=1}^{\infty} \frac{(-1)^{m-1}}{2m-1} \sum_{k=0}^{\infty} \binom{m+k-1}{k} x^{2(k+m)}

x 1 x 2 sin 1 x = r = 1 x 2 r m = 1 r Γ ( r ) Γ ( m ) Γ ( r m + 1 ) ( 2 m 1 ) \displaystyle \frac{x}{\sqrt{1-x^2}}\sin^{-1}x = \sum_{r=1}^{\infty}x^{2r} \sum_{m=1}^{r\Gamma(r)}{\Gamma(m)\Gamma(r-m+1)(2m-1)}

where co-efficient of x 2 r x^{2r} is half of j = 1 ( 2 x ) 2 j j ( 2 j j ) \displaystyle \sum_{j=1}^{\infty} \frac{(2x)^{2j}}{j\binom{2j}{j}} i.e.

r ( 2 r r ) v = 0 r 1 ( 1 ) v ( r 1 v ) 1 2 v + 1 = 2 2 r 1 \displaystyle r\binom{2r}{r} \sum_{v=0}^{r-1} (-1)^v \binom{r-1}{v}\frac{1}{2v+1} = 2^{2r-1}

To verify rewrite as ,

r ( 2 r r ) v = 0 r 1 ( 1 ) v ( r 1 v ) 1 2 v + 1 = r ( 2 r r ) 0 1 v = 0 r 1 ( 1 ) v ( r 1 v ) y 2 v d y = r ( 2 r r ) 0 1 ( 1 y 2 ) r 1 d y \displaystyle r\binom{2r}{r} \sum_{v=0}^{r-1} (-1)^v \binom{r-1}{v}\frac{1}{2v+1} = r\binom{2r}{r} \int_{0}^{1}\sum_{v=0}^{r-1} (-1)^v \binom{r-1}{v} y^{2v} dy = r\binom{2r}{r} \int_{0}^{1} (1-y^2)^{r-1}dy

Hence , r ( 2 r r ) v = 0 r 1 ( 1 ) v ( r 1 v ) 1 2 v + 1 = r ( 2 r r ) 0 1 ( s i n θ ) 2 r 1 d θ \displaystyle r\binom{2r}{r} \sum_{v=0}^{r-1} (-1)^v \binom{r-1}{v}\frac{1}{2v+1} = r\binom{2r}{r} \int_{0}^{1} (sin\theta)^{2r-1} d\theta

Using Walli's integral , r ( 2 r r ) v = 0 r 1 ( 1 ) v ( r 1 v ) 1 2 v + 1 = 2 2 r 1 \displaystyle r\binom{2r}{r} \sum_{v=0}^{r-1} (-1)^v \binom{r-1}{v}\frac{1}{2v+1}=2^{2r-1}

So we are verified with the co-efficient equating task and hence j = 1 ( 2 x ) 2 j j ( 2 j j ) = 2 x 1 x 2 sin 1 x \displaystyle \color{#20A900}{\sum_{j=1}^{\infty} \frac{(2x)^{2j}}{j\binom{2j}{j}} = \frac{2x}{\sqrt{1-x^2}}\sin^{-1}x}

Dividing by 2 x 2x and integrating from 0 0 to x x twice on the green sum we get,

m = 1 ( 2 x ) 2 m m 3 ( 2 m m ) = 4 0 x ( sin 1 y ) 2 y d y \displaystyle \sum_{m=1}^{\infty} \frac{(2x)^{2m}}{m^3\binom{2m}{m}}\ = 4\int_{0}^{x} \frac{(\sin^{-1}y)^2}{y} dy

If we put x = 1 2 \displaystyle x=\frac{1}{\sqrt{2}} then the equation becomes,

S 2 = 4 0 1 2 ( sin 1 y ) 2 y d y \displaystyle \frac{S}{2} = 4\int_{0}^{\frac{1}{\sqrt{2}}} \frac{(\sin^{-1}y)^2}{y} dy , where S S is our required sum

R.H.S is a representation of Log-Sine Integral which we can use as a standard result or even evaluate ,

Using IBP on RHS we get the second term as a Log-Sine integral 0 π 2 t ln sin t 2 d t \displaystyle \int_{0}^{\frac{\pi}{2}} t\ln|\sin\frac{t}{2}| dt where we could make a substitution t 2 t \displaystyle t\to 2t to get upper limit π \pi

We are missing a 2 '2' inside the Logarithm but that doesn't create any difference as we are not using any standard formulae for the evaluation , So let us consider a function ,

F ( a , b ) = 0 π ( sin z ) a cos b z d z = 2 a π Γ ( a + 1 ) cos b π 2 Γ ( a + b + 2 2 ) Γ ( a b + 2 2 ) \displaystyle F(a,b) = \int_{0}^{\pi} (\sin z)^{a}\cos bz dz = \frac{2^{-a}\pi\Gamma(a+1)\cos\frac{b\pi}{2}}{\Gamma(\frac{a+b+2}{2})\Gamma(\frac{a-b+2}{2})}

So our integral equals 2 F ( a , b ) a b ] a=b=1 \displaystyle \frac{\partial^2 F(a,b)}{\partial a \partial b}]_{\text{a=b=1}} which can now be calculated using Trigamma & Catalan's constant.

Putting all the values successively which is lengthy and an easy exercise we get S 8 = π G 4 35 64 ζ ( 3 ) + π 2 32 ln 2 \displaystyle \frac{S}{8} = \frac{\pi G}{4} -\frac{35}{64}\zeta(3) + \frac{\pi^2}{32}\ln2

n = 0 2 n + 1 n ! 2 ( 2 n + 1 ) ! ( n + 1 ) 2 = 2 π G 35 8 ζ ( 3 ) + π 2 4 ln 2 \displaystyle \sum _{ n=0 }^{ \infty }{ \dfrac { { 2 }^{ n+1 }n!^{ 2 } }{ (2n+1)!(n+1)^{ 2 } } } = 2\pi G - \frac{35}{8}\zeta(3) +\frac{\pi^2}{4}\ln 2

Thus making the answer 2 + 35 + 8 + 3 + 2 + 4 + 2 = 56 \boxed{2+35+8+3+2+4+2=56}

I love your solutions!! :)

Hamza A - 5 years ago

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And I love your questions too :)) !!

The most amazing thing i have ever seen, veey nice and neat solution.

Hana Wehbi - 5 years ago

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