MHB How do I solve for a_3 in a complex Fourier series?

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The discussion focuses on solving for the fourth coefficient, a_3, in a discrete time Fourier series where the existing coefficients are given as {3, 1-2j, -1, ?}. The user seeks assistance in determining both the real and imaginary parts of a_3. A proposed method involves rewriting the equation to isolate a_3, leading to the expression a_3 = [sum - (1-2j)e^(jπ/2n) - 3 - (-1)^(n+1)]e^(jπ/2n). Clarification is requested regarding the known value of "sum" that the equation should equal. The conversation emphasizes the need for a complete equation to effectively solve for a_3.
Tan Thom
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Good morning,

I am working on a problem where I am finding the 4th Coefficient in a sample of 4 discrete time Fourier Series coefficients. I got the sum but now I have to solve for a_3 which consists of a real and imaginary part. Any assitance on how to solve for the a_3? Thank you.

$a_k = \{3, 1-2j, -1, ?\}$

Step 1: $(1-2j)e^{j*.5\pi*n} +a_3 e ^ {-(j*.5\pi*n)} + 3 + (-1)^{n+1} $

Step 2: $[(1-2j)(\cos \frac\pi2 n + j \sin \frac\pi2n) + a_3 (\cos \frac\pi2n-j \sin \frac\pi2n)]$
 
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Tan Tom said:
Good morning,

I am working on a problem where I am finding the 4th Coefficient in a sample of 4 discrete time Fourier Series coefficients. I got the sum but now I have to solve for a_3 which consists of a real and imaginary part. Any assitance on how to solve for the a_3? Thank you.

$a_k = \{3, 1-2j, -1, ?\}$

Step 1: $(1-2j)e^{j*.5\pi*n} +a_3 e ^ {-(j*.5\pi*n)} + 3 + (-1)^{n+1} $

Step 2: $[(1-2j)(\cos \frac\pi2 n + j \sin \frac\pi2n) + a_3 (\cos \frac\pi2n-j \sin \frac\pi2n)]$

Hi Tan Tom, welcome to MHB! ;)

If I understand correctly, you have
$$(1-2j)e^{j\frac\pi 2 n} +a_3 e ^ {-(j\frac\pi 2n)} + 3 + (-1)^{n+1}=sum$$
for some known $sum$.

We can rewrite it as:
$$a_3 e ^ {-(j\frac\pi 2 n)}=sum-(1-2j)e^{j\frac \pi 2n} - 3 - (-1)^{n+1}\\
a_3 =\big[sum-(1-2j)e^{j\frac \pi 2n} - 3 - (-1)^{n+1}\big]e ^ {j\frac\pi 2 n}$$
Is that what you're looking for, or am I misunderstanding something?
 
Before you can "solve" you have to have an equation! What is that supposed to be equal to? Klaas van Aarsen is assuming it is to be equal to some number he is calling "sum". Is that correct?
 
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