THEOREM 9.5. Let X be a Hilbert space and let A: X₁, X₂, ... be an orthonormal set in X. Then, if EF (n = 1, 2, ...), we have (1) Σ1%* converges if and only if Σ=₁ |α₂|² < ∞, and (2) if Σ₁%,* converges to x, then α = (x, xn). =1 Proof (2). Consider makking from *x to F x = 2 an n=1 S₂ = 24x₁. α; = (Sn, xj). Since this relationship is true (that is, the value remains the same) for any n> j, it must also be true in the limit, and we can write and the partial sum For n>j, we define Request explain the underlined parts Thm 9.2 это Xis an lim(S₁, xj) = αj. n mer product space, Appealing to Theorem 9.2, we can assert the continuity of the inner product mapping which allows us to interchange the operations of limit and inner product e inver product in the above equation, which yields _n,Y) is a continuous (x, xj) = αj, which is the desired result. Thus, knowing that the given series converges to some x, we have a strong relationship between the coefficients in the series and x.

Advanced Engineering Mathematics
10th Edition
ISBN:9780470458365
Author:Erwin Kreyszig
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Chapter2: Second-order Linear Odes
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THEOREM 9.5. Let X be a Hilbert space and let
A: x₁, x₂,...
be an orthonormal set in X. Then, if a F (n = 1, 2, ...), we have (1)
Σ1%,* converges if and only if Σ%=₁ |α₁|² < ∞, and (2) if Σ₁ª* converges
to x, then a, = (x, xn).
Proof (2). Consider
makking from
XXX to F
and the partial sum
For n >j, we define
x = Σ αnxn
n=1
S₁₂ = Σ α₁x₁
dixi
Request explain
the underlined
peerts
αj = (Sn, xj).
Since this relationship is true (that is, the value remains the same) for any n > j, it
must also be true in the limit, and we can write
Thm 9.2
90 X is an
lim(Sn, xj) = α;.
Inner product space, Appealing to Theorem 9.2, we can assert the continuity of the inner product
mapping which allows us to interchange the operations of limit and inner product
in the above equation, which yields
the inner product
(x,y) is a continuous
(x, xj) = αj,
which is the desired result. Thus, knowing that the given series converges to some x,
we have a strong relationship between the coefficients in the series and x.
Transcribed Image Text:THEOREM 9.5. Let X be a Hilbert space and let A: x₁, x₂,... be an orthonormal set in X. Then, if a F (n = 1, 2, ...), we have (1) Σ1%,* converges if and only if Σ%=₁ |α₁|² < ∞, and (2) if Σ₁ª* converges to x, then a, = (x, xn). Proof (2). Consider makking from XXX to F and the partial sum For n >j, we define x = Σ αnxn n=1 S₁₂ = Σ α₁x₁ dixi Request explain the underlined peerts αj = (Sn, xj). Since this relationship is true (that is, the value remains the same) for any n > j, it must also be true in the limit, and we can write Thm 9.2 90 X is an lim(Sn, xj) = α;. Inner product space, Appealing to Theorem 9.2, we can assert the continuity of the inner product mapping which allows us to interchange the operations of limit and inner product in the above equation, which yields the inner product (x,y) is a continuous (x, xj) = αj, which is the desired result. Thus, knowing that the given series converges to some x, we have a strong relationship between the coefficients in the series and x.
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