Riemann ζ \zeta is back again

Calculus Level 3

Let n n be any natural number such that ζ ( n ) \zeta(n) converges and N 2 N \mathbb{N} - 2\mathbb{N} be the set of all odd numbers. Then we can write

k ( N 2 N ) 1 k n ζ ( n ) = x , \sum_{k \in (\mathbb{N} - 2\mathbb{N})} \frac{1}{k^n \zeta(n)} = x,

for some real number x x . About the number x , x, it is true that.

x 1 x^{-1} is always even. x N x \notin \mathbb{N} For some n , x > 1 n, x > 1 For some n , x N n, x \in \mathbb{N}

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1 solution

Guilherme Niedu
Jan 7, 2019

x = 1 ζ ( n ) [ 1 1 n + 1 3 n + 1 5 n + . . . ] \large \displaystyle x = \frac{1}{\zeta(n) } \left [ \frac{1}{1^n} + \frac{1}{3^n} +\frac{1}{5^n} +... \right]

x = 1 ζ ( n ) [ ζ ( n ) ( 1 2 n + 1 4 n + 1 6 n + . . . ) ] \large \displaystyle x = \frac{1}{\zeta(n) } \left [ \zeta(n) - \left (\frac{1}{2^n} + \frac{1}{4^n} +\frac{1}{6^n} +... \right ) \right]

x = 1 ζ ( n ) [ ζ ( n ) 1 2 n ( 1 1 n + 1 2 n + 1 3 n + . . . ) ] \large \displaystyle x = \frac{1}{\zeta(n) } \left [ \zeta(n) - \frac{1}{2^n} \left (\frac{1}{1^n} + \frac{1}{2^n} +\frac{1}{3^n} +... \right ) \right]

x = 1 ζ ( n ) [ ζ ( n ) 1 2 n ζ ( n ) ] \large \displaystyle x = \frac{1}{\zeta(n) } \left [ \zeta(n) - \frac{1}{2^n} \zeta(n) \right]

x = 1 1 2 n \color{#20A900} \boxed{ \large \displaystyle x = 1 - \frac{1}{2^n}}

So, since n 2 n \geq 2 for ζ \zeta to converge, 0 < x < 1 0 < x < 1 , or x N \color{#3D99F6} \boxed { x \notin \mathbb{N} }

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