∫ 0 1 x ln ( 1 + x 4 ) d x = B π A
If for positive integers A , B , the above integral satisfies, submit the value of ϕ ( B − A ) as your answer where ϕ ( s ) denotes the Euler-Totient Function.
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Consider.
I
=
∫
0
1
x
ln
(
1
+
x
p
)
d
x
as,
ln
(
1
+
x
)
=
n
=
1
∑
∞
n
x
n
(
−
1
)
n
+
1
plugging
x
→
x
p
ln
(
1
+
x
p
)
=
n
=
1
∑
∞
n
x
n
p
(
−
1
)
n
+
1
Note that as the integral has the limits from
0
to
1
so the above series is valid.
Hence
I
=
∫
0
1
n
=
1
∑
∞
n
x
x
n
p
(
−
1
)
n
+
1
d
x
=
n
=
1
∑
∞
n
(
−
1
)
n
+
1
∫
0
1
x
n
p
−
1
d
x
=
n
=
1
∑
∞
p
n
2
(
−
1
)
n
+
1
The second step is verified by Fubini-Tonelli's theorem
By the fact that
n
=
1
∑
∞
n
2
(
−
1
)
n
+
1
=
1
2
π
2
Hence,
I
=
1
2
p
π
2
For our case , plug
p
=
4
, so
I
=
4
8
π
2
Clearly,
ϕ
(
4
8
−
2
)
=
ϕ
(
4
6
)
=
2
2
Q.E.D
Use series expansion of l n ( 1 + x ) yields the values as ζ ( 2 ) / 8 = I = π 2 / 4 8
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