Calculus problem

Calculus Level 5

I n = π π 1 1 + 2 sin ( x / 2 ) ( sin ( n x / 2 ) sin ( x / 2 ) ) 2 d x \large I_n =\int_{-\pi}^\pi \dfrac1{1 + 2^{\sin(x/2)}} \left( \dfrac{\sin(nx/2)}{\sin(x/2)} \right)^2 \, dx

For n = 0 , 1 , 2 , 3 , n=0,1,2,3,\ldots , we define I n I_n as above.

How many of these choices are true?

Choice number 1 : I n = I n + 1 I_n = I_{n+1} for all n 1 n\leq 1 .
Choice number 2 : I 0 , I 1 , I 2 , , I n I_0, I_1, I_2, \ldots,I_n forms an arithmetic progression.
Choice number 3 : m = 0 9 I 2 m = 90 π \displaystyle \sum_{m=0}^9 I_{2m} = 90\pi .
Choice number 4 : m = 0 10 I m = 55 π \displaystyle \sum_{m=0}^{10} I_m = 55\pi .

Submit your answer as the sum of values that represent the answer choice.

For example, if you think that choice number 1, 2, and 4 are true, submit your answer as 1 + 2 + 4 = 7 1+2+4=7 .


The answer is 9.

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1 solution

Mark Hennings
Feb 24, 2016

These integrals are Cesaro Fourier sums for the function f ( x ) = ( 1 + 2 sin 1 2 x ) 1 f(x) \,=\, \big(1 + 2^{\sin\frac12x}\big)^{-1} , without a simple scale factor. Since n = 0 N 1 m = n n e i m x = ( sin 1 2 N x sin 1 2 x ) 2 , \sum_{n=0}^{N-1} \sum_{m=-n}^n e^{imx} \; = \; \left(\frac{\sin \frac12Nx}{\sin\frac12x}\right)^2 \;, with the variable substitution x x x \mapsto -x , we see that (since f ( x ) + f ( x ) = 1 f(x) + f(-x) = 1 ): I N = π π f ( x ) ( sin 1 2 N x sin 1 2 x ) 2 d x = π π f ( x ) ( sin 1 2 N x sin 1 2 x ) 2 d x = 1 2 π π [ f ( x ) + f ( x ) ] ( sin 1 2 N x sin 1 2 x ) 2 d x = 1 2 π π ( sin 1 2 N x sin 1 2 x ) 2 d x = 1 2 n = 0 N 1 m = n n π π e i m x d x = 1 2 n = 0 N 1 m = n n 2 π δ m , 0 = N π \begin{array}{rcl} \displaystyle I_N \; = \; \int_{-\pi}^\pi f(x) \left(\frac{\sin \frac12Nx}{\sin\frac12x}\right)^2\,dx & = & \displaystyle \int_{-\pi}^\pi f(-x) \left(\frac{\sin \frac12Nx}{\sin\frac12x}\right)^2\,dx \\ & =& \displaystyle \tfrac12\int_{-\pi}^\pi\big[f(x) + f(-x)\big] \left(\frac{\sin \frac12Nx}{\sin\frac12x}\right)^2\,dx \\ & = & \displaystyle \tfrac12 \int_{-\pi}^\pi \left(\frac{\sin \frac12Nx}{\sin\frac12x}\right)^2\,dx \\ & = & \displaystyle \tfrac12 \sum_{n=0}^{N-1} \sum_{m=-n}^n \int_{-\pi}^\pi e^{imx}\,dx \\ & = & \displaystyle \tfrac12 \sum_{n=0}^{N-1} \sum_{m=-n}^n 2\pi \delta_{m,0} \\ & = & N\pi \end{array} for all integers N 1 N \ge 1 , where δ m , n \delta_{m,n} is the Kronecker delta. From this it is clear that statement 1 1 is false (should the inequality be the other way around?), but that statements 2 , 3 , 4 2,3,4 are true, making the answer 2 + 3 + 4 = 9 2+3+4 = \boxed{9} .

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