I n = ∫ − π π 1 + 2 sin ( x / 2 ) 1 ( sin ( x / 2 ) sin ( n x / 2 ) ) 2 d x
For n = 0 , 1 , 2 , 3 , … , we define I n as above.
How many of these choices are true?
Choice number 1
:
I
n
=
I
n
+
1
for all
n
≤
1
.
Choice number 2
:
I
0
,
I
1
,
I
2
,
…
,
I
n
forms an arithmetic progression.
Choice number 3
:
m
=
0
∑
9
I
2
m
=
9
0
π
.
Choice number 4
:
m
=
0
∑
1
0
I
m
=
5
5
π
.
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 .
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These integrals are Cesaro Fourier sums for the function f ( x ) = ( 1 + 2 sin 2 1 x ) − 1 , without a simple scale factor. Since n = 0 ∑ N − 1 m = − n ∑ n e i m x = ( sin 2 1 x sin 2 1 N x ) 2 , with the variable substitution x ↦ − x , we see that (since f ( x ) + f ( − x ) = 1 ): I N = ∫ − π π f ( x ) ( sin 2 1 x sin 2 1 N x ) 2 d x = = = = = = ∫ − π π f ( − x ) ( sin 2 1 x sin 2 1 N x ) 2 d x 2 1 ∫ − π π [ f ( x ) + f ( − x ) ] ( sin 2 1 x sin 2 1 N x ) 2 d x 2 1 ∫ − π π ( sin 2 1 x sin 2 1 N x ) 2 d x 2 1 n = 0 ∑ N − 1 m = − n ∑ n ∫ − π π e i m x d x 2 1 n = 0 ∑ N − 1 m = − n ∑ n 2 π δ m , 0 N π for all integers N ≥ 1 , where δ m , n is the Kronecker delta. From this it is clear that statement 1 is false (should the inequality be the other way around?), but that statements 2 , 3 , 4 are true, making the answer 2 + 3 + 4 = 9 .