Connect the curve 4

Calculus Level 5

The figure shows the curves of y = cos ( cos ( x ) ) y=\cos { \left( \cos { \left( x \right) } \right) } in blue and y = sin ( sin ( x ) ) y=\sin { \left( \sin { \left( x \right) } \right) } in red.

It also shows a green region enclosed between these graphs in domain [ 0 , 2 π ] \left[ 0,2\pi \right] .

If the area of the green region is A π B J α ( n ) A{ \pi }^{ B }{ J }_{ \alpha }\left( n \right)

Find A + B + α + n A+B+\alpha+n

Details and Assumptions

  1. J α ( n ) { J }_{ \alpha }\left( n \right) is a Bessel function of the first kind.

Or J α ( n ) = m = 0 ( 1 ) m m ! Γ ( m + α + 1 ) ( n 2 ) 2 m + α J_{ \alpha }(n)=\sum _{ m=0 }^{ \infty } \frac { (-1)^{ m } }{ m!\, \Gamma (m+\alpha +1) } { \left( \frac { n}{ 2 } \right) }^{ 2m+\alpha }

  1. A A , B B , α \alpha and n n are integers.

  2. α < n \alpha<n


The answer is 4.

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2 solutions

Rajdeep Dhingra
May 1, 2015

Let the Green shaded Region's Area be A
A = Area(1) - Area(2) + Area(3) - Area(4) A \text{ = Area(1) - Area(2) + Area(3) - Area(4)} (There is a -ve sign before Area(4) as it is negative but we need to add it.)
We can rewrite this as
A = 0 π c o s ( c o s x ) d x 0 π s i n ( s i n ( x ) ) + π 2 π c o s ( c o s ( x ) ) d x π 2 π s i n ( s i n x ) d x A = \displaystyle \int_{0}^{\pi} {cos(cos{x})dx} - \int_{0}^{\pi} {sin(sin(x))} + \int_{\pi}^{2\pi}{cos(cos(x))dx} \\ \displaystyle - \int_{\pi}^{2\pi}{sin(sin{x})dx}


Changing limits of the integrals we get
A = 0 π c o s ( c o s x ) d x 0 π s i n ( s i n ( x ) ) + 0 π c o s ( c o s ( x ) ) d x + 0 π s i n ( s i n x ) d x A = \displaystyle \int_{0}^{\pi} {cos(cos{x})dx} - \int_{0}^{\pi} {sin(sin(x))} + \int_{0}^{\pi}{cos(cos(x))dx} \\ \displaystyle + \int_{0}^{\pi}{sin(sin{x})dx}

Simplifying we get
A = 2 0 π c o s ( c o s x ) d x A = \displaystyle 2\int_{0}^{\pi}{cos(cos{x})dx}

Hence
A = 2 π J α ( n ) A = \displaystyle 2\pi J_{\alpha} (n)

This is the first time I'm seeing a solution of yours . Nicely done!

A Former Brilliant Member - 6 years, 1 month ago

@Calvin Lin @Archit Boobna
How to evaluate 0 π c o s ( c o s x ) d x \displaystyle \int_{0}^{\pi}{cos(cos{x})dx} without using W|A.

Rajdeep Dhingra - 6 years, 1 month ago

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Since it is already mentioned in the question that the answer has to mentioned in terms of bessel function, it can be done easily.

Actually this integral can't be expressed normally.

2 basic ways are that J0(1) is one of the most well know bessel function, one may know that 1 π 0 π cos cos x d x = J 0 ( 1 ) \frac { 1 }{ \pi } \int _{ 0 }^{ \pi }{ \cos { \cos { x } } } dx={ J }_{ 0 }\left( 1 \right)

or it may be known that 1 π 0 π cos ( n cos x ) d x = J 0 ( n ) \frac { 1 }{ \pi } \int _{ 0 }^{ \pi }{ \cos ({ n \cos { x }) } } dx={ J }_{ 0 }\left( n \right) which can directly lead to the answer, if one doesnt know these, then I am not sure how he'll do it..

Maybe it can be done by immense calculus , and by using the summation.

Archit Boobna - 6 years, 1 month ago

I think there is a typo in your solution.

Why have you multiplied Area(1) and Area(2)

Archit Boobna - 6 years, 1 month ago

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Sorry Typo corrected

Rajdeep Dhingra - 6 years, 1 month ago

@Calvin Lin , Sir I want to take a screenshot of a moving desmos graph on my windows 8 laptop so that I can post it in my next question. Can you tell me how to save a screenshot as GIF? Thanks.

Archit Boobna - 6 years, 1 month ago

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If you send me the link of the graph I can help you.

Rajdeep Dhingra - 6 years, 1 month ago

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I will send you when I will make it.

Archit Boobna - 6 years, 1 month ago

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@Archit Boobna Are you sending me the link ?

Rajdeep Dhingra - 6 years, 1 month ago

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@Rajdeep Dhingra Yes wait I am sending

Archit Boobna - 6 years, 1 month ago

@Rajdeep Dhingra https://www.desmos.com/calculator/n7ifsogxqw

Archit Boobna - 6 years, 1 month ago

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@Archit Boobna Imgur Imgur

Copy the Image link

Rajdeep Dhingra - 6 years, 1 month ago
Danish Mohammed
May 2, 2015

I used the Residue theorem.

The area is A = 0 2 π ( cos ( cos ( x ) ) sin ( sin ( x ) ) ) d x \displaystyle A = \int_{0}^{2\pi} (\cos(\cos(x)) - \sin(\sin(x))) d x . . . (i)

Using the property a b f ( x ) d x = a b f ( a + b x ) d x \displaystyle \int_{a}^{b} f(x) dx = \int_{a}^{b} f(a+b-x) dx ,we get

A = 0 2 π ( cos ( cos ( x ) ) + sin ( sin ( x ) ) ) d x \displaystyle A=\int_{0}^{2\pi} (\cos(\cos(x))+\sin(\sin(x)))dx . . . (ii).

Adding (i) and (ii), we get A = 0 2 π cos ( cos ( x ) ) d x \displaystyle A = \int_{0}^{2\pi} \cos(\cos(x)) dx

Now take z = e i x \displaystyle z=e^{ix} and d z i z = d x \displaystyle \frac{dz}{iz} = dx

A = cos ( z + ( 1 / z ) 2 ) d z i z = 1 i 1 z cos ( z 2 + 1 2 z ) d z \displaystyle A=\oint \cos(\frac{z+(1/z)}{2}) \frac{dz}{iz} = \frac{1}{i} \oint \frac{1}{z} \cos(\frac{z^2+1}{2z}) dz

The contour is the unit circle centred at the origin and is traversed in the counterclockwise (anticlockwise) direction.

There is only one pole of the integrand inside this contour, namely 0.

Hence we find the residue of the integrand 1 z cos ( z 2 + 1 2 z ) \displaystyle\frac{1}{z} \cos(\frac{z^2+1}{2z}) at z = 0 \displaystyle z=0 .

We know that the residue at a point c \displaystyle c is the ( z c ) 1 \displaystyle (z-c)^{-1} coefficient in the laurent series about that point

Writing down the laurent series of the integrand about 0, we find the z 1 \displaystyle z^{-1} coefficient (i.e. the residue) to be

m = 0 ( 1 ) m ( 2 m m ) 2 2 m ( 2 m ) ! \displaystyle \sum_{m=0}^{\infty} \frac{(-1)^m {2m \choose m}}{2^{2m} (2m)!}

= m = 0 ( 1 ) m 2 2 m ( m ! ) ( m ! ) \displaystyle=\sum_{m=0}^{\infty} \frac{(-1)^m}{2^{2m} (m!)(m!)}

= m = 0 ( 1 ) m m ! Γ ( m + 0 + 1 ) ( 1 2 ) 2 m + 0 \displaystyle=\sum_{m=0}^{\infty} \frac{(-1)^m}{m! \Gamma(m+0+1)} (\frac{1}{2})^{2m+0}

= J 0 ( 1 ) \displaystyle=J_{0} (1)

Hence by the residue theorem,

A = 1 i 2 π i ( z 0 P R e s z = z 0 ( 1 z cos ( z 2 + 1 2 z ) ) ) \displaystyle A = \frac{1}{i} 2\pi i(\sum_{z_0 \in P} \mathrm{Res}_{z=z_0}(\frac{1}{z} \cos(\frac{z^2+1}{2z}))) where P \displaystyle P is the set of all poles inside the contour

A = 2 π R e s z = 0 ( 1 z cos ( z 2 + 1 2 z ) ) = 2 π J 0 ( 1 ) \displaystyle A = 2\pi \mathrm{Res}_{z=0} (\frac{1}{z} \cos(\frac{z^2+1}{2z})) = \boxed{2 \pi J_{0}(1)}

So finally we have A = 2 , B = 1 , α = 0 , n = 1 \displaystyle A=2, B=1, \alpha = 0, n=1

A + B + α + n = 4 \displaystyle A+B+\alpha+n=\boxed{4}

You sir, are a genius.

Archit Boobna - 6 years, 1 month ago

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Haha. I'm no genius. But thanks. :)

Danish Mohammed - 6 years, 1 month ago

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