We have to prove that C 1 [ 0 , 1 ] C^1[0,1] C 1 [ 0 , 1 ] is not complete with the norm:
∣ ∣ f ∣ ∣ ∞ = s u p x ∈ [ 0 , 1 ] ∣ f ( x ) ∣ ||f||_{\infty}=sup_{x \in [0,1]}|f(x)| ∣∣ f ∣ ∣ ∞ = s u p x ∈ [ 0 , 1 ] ∣ f ( x ) ∣
The right sequence for norm is f n = x + 1 x f_n=\sqrt{x+\dfrac{1}{x}} f n = x + x 1
Notice that n ∈ N : f n ∈ C 1 [ 0 , 1 ] n\in N:f_n\in C^1[0,1] n ∈ N : f n ∈ C 1 [ 0 , 1 ]
let f = x f=\sqrt{x} f = x
We see that f n f_n f n converges to fin sup norm in C [ 0 , 1 ] C[0,1] C [ 0 , 1 ] , Thus it is cauchy.
C 1 [ 0 , 1 ] C^1[0,1] C 1 [ 0 , 1 ] is a supspace of C [ 0 , 1 ] C[0,1] C [ 0 , 1 ] and all terms of ( f n ) (f_n) ( f n ) are in C 1 [ 0 , 1 ] C^1[0,1] C 1 [ 0 , 1 ] ,So
As the f n f_n f n converges to C [ 0 , 1 ] C[0,1] C [ 0 , 1 ]
⟹ ( f n ) is Cauchy in C 1 [ 0 , 1 ] . \implies (f_n) \text{ is Cauchy in } C^1[0,1]. ⟹ ( f n ) is Cauchy in C 1 [ 0 , 1 ] .
So Given space is not complete with norm ∣ ∣ f ( x ) ∣ ∣ ∞ = s u p x ∈ [ 0 , 1 ] ∣ f ( x ) ∣ ||f(x)||_{\infty}=sup_{x\in[0,1]}|f(x)| ∣∣ f ( x ) ∣ ∣ ∞ = s u p x ∈ [ 0 , 1 ] ∣ f ( x ) ∣
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