How did we come up with this other form for this sum?
$begingroup$
Could someone please tell how is it possible to change this sum :
$${x_{3}(m)=frac{1}{N}sum_{k=0}^{N-1}bigg[sum_{n=0}^{N-1}x_{1}(n)e^{-j2pi kfrac{n}{N}}bigg ]bigg[sum_{l=0}^{N-1}x_{2}(l)e^{-j2pi kfrac{l}{N}}bigg ] e^{j2pi kfrac{m}{N}}} $$
to :
$$ frac{1}{N}sum_{n=0}^{N-1}x_{1}(n)sum_{l=0}^{N-1}x_{2}(l)bigg[sum_{k=0}^{N-1}e^{j2pi kfrac{m-n-l}{N}}bigg ]$$
?
sequences-and-series
$endgroup$
add a comment |
$begingroup$
Could someone please tell how is it possible to change this sum :
$${x_{3}(m)=frac{1}{N}sum_{k=0}^{N-1}bigg[sum_{n=0}^{N-1}x_{1}(n)e^{-j2pi kfrac{n}{N}}bigg ]bigg[sum_{l=0}^{N-1}x_{2}(l)e^{-j2pi kfrac{l}{N}}bigg ] e^{j2pi kfrac{m}{N}}} $$
to :
$$ frac{1}{N}sum_{n=0}^{N-1}x_{1}(n)sum_{l=0}^{N-1}x_{2}(l)bigg[sum_{k=0}^{N-1}e^{j2pi kfrac{m-n-l}{N}}bigg ]$$
?
sequences-and-series
$endgroup$
add a comment |
$begingroup$
Could someone please tell how is it possible to change this sum :
$${x_{3}(m)=frac{1}{N}sum_{k=0}^{N-1}bigg[sum_{n=0}^{N-1}x_{1}(n)e^{-j2pi kfrac{n}{N}}bigg ]bigg[sum_{l=0}^{N-1}x_{2}(l)e^{-j2pi kfrac{l}{N}}bigg ] e^{j2pi kfrac{m}{N}}} $$
to :
$$ frac{1}{N}sum_{n=0}^{N-1}x_{1}(n)sum_{l=0}^{N-1}x_{2}(l)bigg[sum_{k=0}^{N-1}e^{j2pi kfrac{m-n-l}{N}}bigg ]$$
?
sequences-and-series
$endgroup$
Could someone please tell how is it possible to change this sum :
$${x_{3}(m)=frac{1}{N}sum_{k=0}^{N-1}bigg[sum_{n=0}^{N-1}x_{1}(n)e^{-j2pi kfrac{n}{N}}bigg ]bigg[sum_{l=0}^{N-1}x_{2}(l)e^{-j2pi kfrac{l}{N}}bigg ] e^{j2pi kfrac{m}{N}}} $$
to :
$$ frac{1}{N}sum_{n=0}^{N-1}x_{1}(n)sum_{l=0}^{N-1}x_{2}(l)bigg[sum_{k=0}^{N-1}e^{j2pi kfrac{m-n-l}{N}}bigg ]$$
?
sequences-and-series
sequences-and-series
edited Dec 3 '18 at 13:12
Hilbert
asked Dec 3 '18 at 10:12
HilbertHilbert
1479
1479
add a comment |
add a comment |
1 Answer
1
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$begingroup$
begin{align}&frac{1}{N}sum_{k=0}^{N-1}bigg[sum_{n=0}^{N-1}x_{1}(n)e^{-j2pi kfrac{n}{N}}bigg ]bigg[sum_{l=0}^{N-1}x_{2}(l)e^{-j2pi kfrac{l}{N}}bigg ] e^{j2pi kfrac{m}{N}}\
&=frac{1}{N}sum_{k=0}^{N-1}sum_{n=0}^{N-1}sum_{l=0}^{N-1}x_{1}(n)e^{-j2pi kfrac{n}{N}}x_{2}(l)e^{-j2pi kfrac{l}{N}} e^{j2pi kfrac{m}{N}} text{, distributive law}\
&=frac{1}{N}sum_{k=0}^{N-1}sum_{n=0}^{N-1}sum_{l=0}^{N-1}x_{1}(n)x_{2}(l) e^{j2pi kfrac{m-n-l}{N}} \
&=frac{1}{N}sum_{n=0}^{N-1}sum_{l=0}^{N-1}sum_{k=0}^{N-1}x_{1}(n)x_{2}(l) e^{j2pi kfrac{m-n-l}{N}} \
&=frac{1}{N}sum_{n=0}^{N-1}x_{1}(n)sum_{l=0}^{N-1}x_{2}(l) sum_{k=0}^{N-1}e^{j2pi kfrac{m-n-l}{N}} \
end{align}
Notice that we are dealing with finite sum, the order of summation doesn't matter.
$endgroup$
$begingroup$
can you please specify how did you use the distributive law in going from :$\ frac{1}{N}sum_{k=0}^{N-1}bigg[sum_{n=0}^{N-1}x_{1}(n)e^{-j2pi kfrac{n}{N}}bigg ]bigg[sum_{l=0}^{N-1}x_{2}(l)e^{-j2pi kfrac{l}{N}}bigg ] e^{j2pi kfrac{m}{N}} $ to : $\ frac{1}{N}sum_{k=0}^{N-1}sum_{n=0}^{N-1}sum_{l=0}^{N-1}x_{1}(n)e^{-j2pi kfrac{n}{N}}x_{2}(l)e^{-j2pi kfrac{l}{N}} e^{j2pi kfrac{m}{N}} \$ Because I think this is really the key step .
$endgroup$
– Hilbert
Dec 3 '18 at 16:14
1
$begingroup$
For example $(a+b+c)(d+e+f)=ad+ae+af+bd+be+bf+cd+cd+cf$. Product of sum is the sum of the product.
$endgroup$
– Siong Thye Goh
Dec 3 '18 at 16:19
$begingroup$
Thank you now I can see through it !
$endgroup$
– Hilbert
Dec 3 '18 at 16:22
add a comment |
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$begingroup$
begin{align}&frac{1}{N}sum_{k=0}^{N-1}bigg[sum_{n=0}^{N-1}x_{1}(n)e^{-j2pi kfrac{n}{N}}bigg ]bigg[sum_{l=0}^{N-1}x_{2}(l)e^{-j2pi kfrac{l}{N}}bigg ] e^{j2pi kfrac{m}{N}}\
&=frac{1}{N}sum_{k=0}^{N-1}sum_{n=0}^{N-1}sum_{l=0}^{N-1}x_{1}(n)e^{-j2pi kfrac{n}{N}}x_{2}(l)e^{-j2pi kfrac{l}{N}} e^{j2pi kfrac{m}{N}} text{, distributive law}\
&=frac{1}{N}sum_{k=0}^{N-1}sum_{n=0}^{N-1}sum_{l=0}^{N-1}x_{1}(n)x_{2}(l) e^{j2pi kfrac{m-n-l}{N}} \
&=frac{1}{N}sum_{n=0}^{N-1}sum_{l=0}^{N-1}sum_{k=0}^{N-1}x_{1}(n)x_{2}(l) e^{j2pi kfrac{m-n-l}{N}} \
&=frac{1}{N}sum_{n=0}^{N-1}x_{1}(n)sum_{l=0}^{N-1}x_{2}(l) sum_{k=0}^{N-1}e^{j2pi kfrac{m-n-l}{N}} \
end{align}
Notice that we are dealing with finite sum, the order of summation doesn't matter.
$endgroup$
$begingroup$
can you please specify how did you use the distributive law in going from :$\ frac{1}{N}sum_{k=0}^{N-1}bigg[sum_{n=0}^{N-1}x_{1}(n)e^{-j2pi kfrac{n}{N}}bigg ]bigg[sum_{l=0}^{N-1}x_{2}(l)e^{-j2pi kfrac{l}{N}}bigg ] e^{j2pi kfrac{m}{N}} $ to : $\ frac{1}{N}sum_{k=0}^{N-1}sum_{n=0}^{N-1}sum_{l=0}^{N-1}x_{1}(n)e^{-j2pi kfrac{n}{N}}x_{2}(l)e^{-j2pi kfrac{l}{N}} e^{j2pi kfrac{m}{N}} \$ Because I think this is really the key step .
$endgroup$
– Hilbert
Dec 3 '18 at 16:14
1
$begingroup$
For example $(a+b+c)(d+e+f)=ad+ae+af+bd+be+bf+cd+cd+cf$. Product of sum is the sum of the product.
$endgroup$
– Siong Thye Goh
Dec 3 '18 at 16:19
$begingroup$
Thank you now I can see through it !
$endgroup$
– Hilbert
Dec 3 '18 at 16:22
add a comment |
$begingroup$
begin{align}&frac{1}{N}sum_{k=0}^{N-1}bigg[sum_{n=0}^{N-1}x_{1}(n)e^{-j2pi kfrac{n}{N}}bigg ]bigg[sum_{l=0}^{N-1}x_{2}(l)e^{-j2pi kfrac{l}{N}}bigg ] e^{j2pi kfrac{m}{N}}\
&=frac{1}{N}sum_{k=0}^{N-1}sum_{n=0}^{N-1}sum_{l=0}^{N-1}x_{1}(n)e^{-j2pi kfrac{n}{N}}x_{2}(l)e^{-j2pi kfrac{l}{N}} e^{j2pi kfrac{m}{N}} text{, distributive law}\
&=frac{1}{N}sum_{k=0}^{N-1}sum_{n=0}^{N-1}sum_{l=0}^{N-1}x_{1}(n)x_{2}(l) e^{j2pi kfrac{m-n-l}{N}} \
&=frac{1}{N}sum_{n=0}^{N-1}sum_{l=0}^{N-1}sum_{k=0}^{N-1}x_{1}(n)x_{2}(l) e^{j2pi kfrac{m-n-l}{N}} \
&=frac{1}{N}sum_{n=0}^{N-1}x_{1}(n)sum_{l=0}^{N-1}x_{2}(l) sum_{k=0}^{N-1}e^{j2pi kfrac{m-n-l}{N}} \
end{align}
Notice that we are dealing with finite sum, the order of summation doesn't matter.
$endgroup$
$begingroup$
can you please specify how did you use the distributive law in going from :$\ frac{1}{N}sum_{k=0}^{N-1}bigg[sum_{n=0}^{N-1}x_{1}(n)e^{-j2pi kfrac{n}{N}}bigg ]bigg[sum_{l=0}^{N-1}x_{2}(l)e^{-j2pi kfrac{l}{N}}bigg ] e^{j2pi kfrac{m}{N}} $ to : $\ frac{1}{N}sum_{k=0}^{N-1}sum_{n=0}^{N-1}sum_{l=0}^{N-1}x_{1}(n)e^{-j2pi kfrac{n}{N}}x_{2}(l)e^{-j2pi kfrac{l}{N}} e^{j2pi kfrac{m}{N}} \$ Because I think this is really the key step .
$endgroup$
– Hilbert
Dec 3 '18 at 16:14
1
$begingroup$
For example $(a+b+c)(d+e+f)=ad+ae+af+bd+be+bf+cd+cd+cf$. Product of sum is the sum of the product.
$endgroup$
– Siong Thye Goh
Dec 3 '18 at 16:19
$begingroup$
Thank you now I can see through it !
$endgroup$
– Hilbert
Dec 3 '18 at 16:22
add a comment |
$begingroup$
begin{align}&frac{1}{N}sum_{k=0}^{N-1}bigg[sum_{n=0}^{N-1}x_{1}(n)e^{-j2pi kfrac{n}{N}}bigg ]bigg[sum_{l=0}^{N-1}x_{2}(l)e^{-j2pi kfrac{l}{N}}bigg ] e^{j2pi kfrac{m}{N}}\
&=frac{1}{N}sum_{k=0}^{N-1}sum_{n=0}^{N-1}sum_{l=0}^{N-1}x_{1}(n)e^{-j2pi kfrac{n}{N}}x_{2}(l)e^{-j2pi kfrac{l}{N}} e^{j2pi kfrac{m}{N}} text{, distributive law}\
&=frac{1}{N}sum_{k=0}^{N-1}sum_{n=0}^{N-1}sum_{l=0}^{N-1}x_{1}(n)x_{2}(l) e^{j2pi kfrac{m-n-l}{N}} \
&=frac{1}{N}sum_{n=0}^{N-1}sum_{l=0}^{N-1}sum_{k=0}^{N-1}x_{1}(n)x_{2}(l) e^{j2pi kfrac{m-n-l}{N}} \
&=frac{1}{N}sum_{n=0}^{N-1}x_{1}(n)sum_{l=0}^{N-1}x_{2}(l) sum_{k=0}^{N-1}e^{j2pi kfrac{m-n-l}{N}} \
end{align}
Notice that we are dealing with finite sum, the order of summation doesn't matter.
$endgroup$
begin{align}&frac{1}{N}sum_{k=0}^{N-1}bigg[sum_{n=0}^{N-1}x_{1}(n)e^{-j2pi kfrac{n}{N}}bigg ]bigg[sum_{l=0}^{N-1}x_{2}(l)e^{-j2pi kfrac{l}{N}}bigg ] e^{j2pi kfrac{m}{N}}\
&=frac{1}{N}sum_{k=0}^{N-1}sum_{n=0}^{N-1}sum_{l=0}^{N-1}x_{1}(n)e^{-j2pi kfrac{n}{N}}x_{2}(l)e^{-j2pi kfrac{l}{N}} e^{j2pi kfrac{m}{N}} text{, distributive law}\
&=frac{1}{N}sum_{k=0}^{N-1}sum_{n=0}^{N-1}sum_{l=0}^{N-1}x_{1}(n)x_{2}(l) e^{j2pi kfrac{m-n-l}{N}} \
&=frac{1}{N}sum_{n=0}^{N-1}sum_{l=0}^{N-1}sum_{k=0}^{N-1}x_{1}(n)x_{2}(l) e^{j2pi kfrac{m-n-l}{N}} \
&=frac{1}{N}sum_{n=0}^{N-1}x_{1}(n)sum_{l=0}^{N-1}x_{2}(l) sum_{k=0}^{N-1}e^{j2pi kfrac{m-n-l}{N}} \
end{align}
Notice that we are dealing with finite sum, the order of summation doesn't matter.
answered Dec 3 '18 at 13:35
Siong Thye GohSiong Thye Goh
100k1465117
100k1465117
$begingroup$
can you please specify how did you use the distributive law in going from :$\ frac{1}{N}sum_{k=0}^{N-1}bigg[sum_{n=0}^{N-1}x_{1}(n)e^{-j2pi kfrac{n}{N}}bigg ]bigg[sum_{l=0}^{N-1}x_{2}(l)e^{-j2pi kfrac{l}{N}}bigg ] e^{j2pi kfrac{m}{N}} $ to : $\ frac{1}{N}sum_{k=0}^{N-1}sum_{n=0}^{N-1}sum_{l=0}^{N-1}x_{1}(n)e^{-j2pi kfrac{n}{N}}x_{2}(l)e^{-j2pi kfrac{l}{N}} e^{j2pi kfrac{m}{N}} \$ Because I think this is really the key step .
$endgroup$
– Hilbert
Dec 3 '18 at 16:14
1
$begingroup$
For example $(a+b+c)(d+e+f)=ad+ae+af+bd+be+bf+cd+cd+cf$. Product of sum is the sum of the product.
$endgroup$
– Siong Thye Goh
Dec 3 '18 at 16:19
$begingroup$
Thank you now I can see through it !
$endgroup$
– Hilbert
Dec 3 '18 at 16:22
add a comment |
$begingroup$
can you please specify how did you use the distributive law in going from :$\ frac{1}{N}sum_{k=0}^{N-1}bigg[sum_{n=0}^{N-1}x_{1}(n)e^{-j2pi kfrac{n}{N}}bigg ]bigg[sum_{l=0}^{N-1}x_{2}(l)e^{-j2pi kfrac{l}{N}}bigg ] e^{j2pi kfrac{m}{N}} $ to : $\ frac{1}{N}sum_{k=0}^{N-1}sum_{n=0}^{N-1}sum_{l=0}^{N-1}x_{1}(n)e^{-j2pi kfrac{n}{N}}x_{2}(l)e^{-j2pi kfrac{l}{N}} e^{j2pi kfrac{m}{N}} \$ Because I think this is really the key step .
$endgroup$
– Hilbert
Dec 3 '18 at 16:14
1
$begingroup$
For example $(a+b+c)(d+e+f)=ad+ae+af+bd+be+bf+cd+cd+cf$. Product of sum is the sum of the product.
$endgroup$
– Siong Thye Goh
Dec 3 '18 at 16:19
$begingroup$
Thank you now I can see through it !
$endgroup$
– Hilbert
Dec 3 '18 at 16:22
$begingroup$
can you please specify how did you use the distributive law in going from :$\ frac{1}{N}sum_{k=0}^{N-1}bigg[sum_{n=0}^{N-1}x_{1}(n)e^{-j2pi kfrac{n}{N}}bigg ]bigg[sum_{l=0}^{N-1}x_{2}(l)e^{-j2pi kfrac{l}{N}}bigg ] e^{j2pi kfrac{m}{N}} $ to : $\ frac{1}{N}sum_{k=0}^{N-1}sum_{n=0}^{N-1}sum_{l=0}^{N-1}x_{1}(n)e^{-j2pi kfrac{n}{N}}x_{2}(l)e^{-j2pi kfrac{l}{N}} e^{j2pi kfrac{m}{N}} \$ Because I think this is really the key step .
$endgroup$
– Hilbert
Dec 3 '18 at 16:14
$begingroup$
can you please specify how did you use the distributive law in going from :$\ frac{1}{N}sum_{k=0}^{N-1}bigg[sum_{n=0}^{N-1}x_{1}(n)e^{-j2pi kfrac{n}{N}}bigg ]bigg[sum_{l=0}^{N-1}x_{2}(l)e^{-j2pi kfrac{l}{N}}bigg ] e^{j2pi kfrac{m}{N}} $ to : $\ frac{1}{N}sum_{k=0}^{N-1}sum_{n=0}^{N-1}sum_{l=0}^{N-1}x_{1}(n)e^{-j2pi kfrac{n}{N}}x_{2}(l)e^{-j2pi kfrac{l}{N}} e^{j2pi kfrac{m}{N}} \$ Because I think this is really the key step .
$endgroup$
– Hilbert
Dec 3 '18 at 16:14
1
1
$begingroup$
For example $(a+b+c)(d+e+f)=ad+ae+af+bd+be+bf+cd+cd+cf$. Product of sum is the sum of the product.
$endgroup$
– Siong Thye Goh
Dec 3 '18 at 16:19
$begingroup$
For example $(a+b+c)(d+e+f)=ad+ae+af+bd+be+bf+cd+cd+cf$. Product of sum is the sum of the product.
$endgroup$
– Siong Thye Goh
Dec 3 '18 at 16:19
$begingroup$
Thank you now I can see through it !
$endgroup$
– Hilbert
Dec 3 '18 at 16:22
$begingroup$
Thank you now I can see through it !
$endgroup$
– Hilbert
Dec 3 '18 at 16:22
add a comment |
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