Calculate $sum_{k=l}^{2n} binom{2n+k}{2k} frac{(2k-1)!!}{(k-l)!} (-1)^k$ [closed]
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I'm trying to proof that
$$sum_{k=l}^{2n} binom{2n+k}{2k} frac{(2k-1)!!}{(k-l)!} (-1)^k = begin{cases} 0 quad {rm if} , , l , ,{rm odd} \ frac{(-1)^{n-l/2}(2n+l)!}{4^n left(n-frac{l}{2}right)! left(n+frac{l}{2}right)!} quad {rm if} , , l , ,{rm even} end{cases}$$
where $()!$ is factorial and $()!!$ double-factorial.
Any idea?
edit: Before down-voting it would be more helpful to state what information is missing, but that is probably too much to ask...
summation binomial-coefficients factorial
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closed as off-topic by Davide Giraudo, KReiser, Paul Frost, Lee David Chung Lin, amWhy Dec 28 '18 at 17:37
This question appears to be off-topic. The users who voted to close gave this specific reason:
- "This question is missing context or other details: Please provide additional context, which ideally explains why the question is relevant to you and our community. Some forms of context include: background and motivation, relevant definitions, source, possible strategies, your current progress, why the question is interesting or important, etc." – Davide Giraudo, KReiser, Paul Frost, Lee David Chung Lin, amWhy
If this question can be reworded to fit the rules in the help center, please edit the question.
add a comment |
$begingroup$
I'm trying to proof that
$$sum_{k=l}^{2n} binom{2n+k}{2k} frac{(2k-1)!!}{(k-l)!} (-1)^k = begin{cases} 0 quad {rm if} , , l , ,{rm odd} \ frac{(-1)^{n-l/2}(2n+l)!}{4^n left(n-frac{l}{2}right)! left(n+frac{l}{2}right)!} quad {rm if} , , l , ,{rm even} end{cases}$$
where $()!$ is factorial and $()!!$ double-factorial.
Any idea?
edit: Before down-voting it would be more helpful to state what information is missing, but that is probably too much to ask...
summation binomial-coefficients factorial
$endgroup$
closed as off-topic by Davide Giraudo, KReiser, Paul Frost, Lee David Chung Lin, amWhy Dec 28 '18 at 17:37
This question appears to be off-topic. The users who voted to close gave this specific reason:
- "This question is missing context or other details: Please provide additional context, which ideally explains why the question is relevant to you and our community. Some forms of context include: background and motivation, relevant definitions, source, possible strategies, your current progress, why the question is interesting or important, etc." – Davide Giraudo, KReiser, Paul Frost, Lee David Chung Lin, amWhy
If this question can be reworded to fit the rules in the help center, please edit the question.
2
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I have not down-voted, but perhaps is because you haven't include any information, like how did you encounter this problem? Or how did you obtain the answer?
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– Zacky
Dec 25 '18 at 21:43
add a comment |
$begingroup$
I'm trying to proof that
$$sum_{k=l}^{2n} binom{2n+k}{2k} frac{(2k-1)!!}{(k-l)!} (-1)^k = begin{cases} 0 quad {rm if} , , l , ,{rm odd} \ frac{(-1)^{n-l/2}(2n+l)!}{4^n left(n-frac{l}{2}right)! left(n+frac{l}{2}right)!} quad {rm if} , , l , ,{rm even} end{cases}$$
where $()!$ is factorial and $()!!$ double-factorial.
Any idea?
edit: Before down-voting it would be more helpful to state what information is missing, but that is probably too much to ask...
summation binomial-coefficients factorial
$endgroup$
I'm trying to proof that
$$sum_{k=l}^{2n} binom{2n+k}{2k} frac{(2k-1)!!}{(k-l)!} (-1)^k = begin{cases} 0 quad {rm if} , , l , ,{rm odd} \ frac{(-1)^{n-l/2}(2n+l)!}{4^n left(n-frac{l}{2}right)! left(n+frac{l}{2}right)!} quad {rm if} , , l , ,{rm even} end{cases}$$
where $()!$ is factorial and $()!!$ double-factorial.
Any idea?
edit: Before down-voting it would be more helpful to state what information is missing, but that is probably too much to ask...
summation binomial-coefficients factorial
summation binomial-coefficients factorial
edited Dec 25 '18 at 0:17
Diger
asked Dec 24 '18 at 23:30
DigerDiger
1,7871414
1,7871414
closed as off-topic by Davide Giraudo, KReiser, Paul Frost, Lee David Chung Lin, amWhy Dec 28 '18 at 17:37
This question appears to be off-topic. The users who voted to close gave this specific reason:
- "This question is missing context or other details: Please provide additional context, which ideally explains why the question is relevant to you and our community. Some forms of context include: background and motivation, relevant definitions, source, possible strategies, your current progress, why the question is interesting or important, etc." – Davide Giraudo, KReiser, Paul Frost, Lee David Chung Lin, amWhy
If this question can be reworded to fit the rules in the help center, please edit the question.
closed as off-topic by Davide Giraudo, KReiser, Paul Frost, Lee David Chung Lin, amWhy Dec 28 '18 at 17:37
This question appears to be off-topic. The users who voted to close gave this specific reason:
- "This question is missing context or other details: Please provide additional context, which ideally explains why the question is relevant to you and our community. Some forms of context include: background and motivation, relevant definitions, source, possible strategies, your current progress, why the question is interesting or important, etc." – Davide Giraudo, KReiser, Paul Frost, Lee David Chung Lin, amWhy
If this question can be reworded to fit the rules in the help center, please edit the question.
2
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I have not down-voted, but perhaps is because you haven't include any information, like how did you encounter this problem? Or how did you obtain the answer?
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– Zacky
Dec 25 '18 at 21:43
add a comment |
2
$begingroup$
I have not down-voted, but perhaps is because you haven't include any information, like how did you encounter this problem? Or how did you obtain the answer?
$endgroup$
– Zacky
Dec 25 '18 at 21:43
2
2
$begingroup$
I have not down-voted, but perhaps is because you haven't include any information, like how did you encounter this problem? Or how did you obtain the answer?
$endgroup$
– Zacky
Dec 25 '18 at 21:43
$begingroup$
I have not down-voted, but perhaps is because you haven't include any information, like how did you encounter this problem? Or how did you obtain the answer?
$endgroup$
– Zacky
Dec 25 '18 at 21:43
add a comment |
1 Answer
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We seek to evaluate
$$sum_{k=q}^{2n} {2n+kchoose 2k}
frac{(2k-1)!}{(k-1)! times 2^{k-1}} frac{1}{(k-q)!} (-1)^k
\ = sum_{k=q}^{2n} {2n+kchoose 2k}
frac{(2k)!}{k! times 2^{k}} frac{1}{(k-q)!} (-1)^k
.$$
This is
$$q! sum_{k=q}^{2n} {2n+kchoose 2k}
frac{(2k)!}{k! times k! times 2^{k}}
frac{k!}{q!times(k-q)!} frac{(-1)^k}{2^k}
\ = q! sum_{k=q}^{2n} {2n+kchoose 2k}
{2kchoose k} {kchoose q}
frac{(-1)^k}{2^k}.$$
Observe that
$${2n+kchoose 2k} {2kchoose k}
= frac{(2n+k)!}{(2n-k)! times k! times k!}
= {2n+kchoose 2n} {2nchoose k}$$
and furthermore
$${2nchoose k} {kchoose q}
= frac{(2n)!}{(2n-k)! times q! times (k-q)!}
= {2nchoose q} {2n-qchoose k-q}.$$
We get for the sum
$${2nchoose q} q!
sum_{k=q}^{2n} {2n+kchoose 2n} {2n-qchoose k-q}
frac{(-1)^k}{2^k}
\ = {2nchoose q} q! frac{(-1)^q}{2^q}
sum_{k=0}^{2n-q} {2n+q+kchoose 2n} {2n-qchoose k}
frac{(-1)^k}{2^k}
.$$
This becomes
$${2nchoose q} q! frac{(-1)^q}{2^q}
sum_{k=0}^{2n-q} {2n+q+kchoose 2n} [z^{2n-q-k}] (1+z)^{2n-q}
frac{(-1)^k}{2^k}
\ = {2nchoose q} q! frac{(-1)^q}{2^q}
[z^{2n-q}] (1+z)^{2n-q}
sum_{k=0}^{2n-q} {2n+q+kchoose 2n}
frac{(-1)^k}{2^k} z^k.$$
Now we may extend $k$ beyond $2n-q$ because of the coefficient
extractor $[z^{2n-q}]$ (no contribution) and get
$${2nchoose q} q! frac{(-1)^q}{2^q}
[z^{2n-q}] (1+z)^{2n-q}
sum_{kge 0} {2n+q+kchoose 2n}
frac{(-1)^k}{2^k} z^k
\ = {2nchoose q} q! frac{(-1)^q}{2^q}
[z^{2n-q}] (1+z)^{2n-q} [w^{2n}] (1+w)^{2n+q}
sum_{kge 0} (1+w)^k
frac{(-1)^k}{2^k} z^k
\ = {2nchoose q} q! frac{(-1)^q}{2^q}
[z^{2n-q}] (1+z)^{2n-q} [w^{2n}] (1+w)^{2n+q}
frac{1}{1+z(1+w)/2}.$$
Re-write this as
$${2nchoose q} q! frac{(-1)^q}{2^q}
[w^{2n}] (1+w)^{2n+q}
mathrm{Res}_{z=0} frac{1}{z^{2n-q+1}} (1+z)^{2n-q}
frac{1}{1+z(1+w)/2}.$$
Working with the residue we apply the substitution $z/(1+z) = v$
or $z=v/(1-v)$ to get
$$mathrm{Res}_{v=0} frac{1}{v^{2n-q}}
frac{1-v}{v} frac{1}{1+(v/(1-v))(1+w)/2} frac{1}{(1-v)^2}
\ = mathrm{Res}_{v=0} frac{1}{v^{2n-q+1}}
frac{1}{1-v+v(1+w)/2}
\ = mathrm{Res}_{v=0} frac{1}{v^{2n-q+1}}
frac{1}{1-v(1-w)/2}
= frac{1}{2^{2n-q}} (1-w)^{2n-q}.$$
Substitute into the remaining coefficient extractor to get
$${2nchoose q} q! frac{(-1)^q}{2^q}
[w^{2n}] (1+w)^{2n+q} frac{1}{2^{2n-q}} (1-w)^{2n-q}
\ = {2nchoose q} q! frac{(-1)^q}{2^{2n}}
sum_{p=0}^{2n-q} (-1)^p {2n-qchoose p} {2n+qchoose 2n-p}.$$
Now
$${2nchoose q} {2n-qchoose p}
= frac{(2n)!}{q!times p! times (2n-q-p)!}
= {2nchoose p} {2n-pchoose q}$$
and
$${2n-pchoose q} {2n+qchoose 2n-p}
= frac{(2n+q)!}{q! times (2n-p-q)! times (p+q)!}
= {2n+qchoose q} {2nchoose p+q}.$$
This yields
$${2n+qchoose q} q! frac{(-1)^q}{2^{2n}}
sum_{p=0}^{2n-q} (-1)^p {2nchoose p} {2nchoose p+q}
\ = {2n+qchoose q} q! frac{(-1)^q}{2^{2n}}
sum_{p=0}^{2n-q} (-1)^p {2nchoose p}
[z^{2n-p-q}] (1+z)^{2n}
\ = {2n+qchoose q} q! frac{(-1)^q}{2^{2n}}
[z^{2n-q}] (1+z)^{2n}
sum_{p=0}^{2n-q} (-1)^p {2nchoose p} z^p.$$
Now we may extend $p$ beyond $2n-q$ because of the coeffcient
extractor $[z^{2n-q}]$ in front. We find
$${2n+qchoose q} q! frac{(-1)^q}{2^{2n}}
[z^{2n-q}] (1+z)^{2n}
sum_{pge 0} (-1)^p {2nchoose p} z^p
\ = {2n+qchoose q} q! frac{(-1)^q}{2^{2n}}
[z^{2n-q}] (1+z)^{2n} (1-z)^{2n}
\ = {2n+qchoose q} q! frac{(-1)^q}{2^{2n}}
[z^{2n-q}] (1-z^2)^{2n}.$$
Concluding we immediately obtain zero when $q$ is odd, and otherwise
we find
$${2n+qchoose q} q! frac{(-1)^q}{2^{2n}}
[z^{2(n-q/2)}] (1-z^2)^{2n}
\ = {2n+qchoose q} q! frac{(-1)^q}{2^{2n}}
[z^{n-q/2}] (1-z)^{2n}.$$
This is
$${2n+qchoose q} q! frac{(-1)^q}{2^{2n}}
(-1)^{n-q/2} {2nchoose n-q/2}$$
or alternatively
$$bbox[5px,border:2px solid #00A000]{
frac{(-1)^{n+q/2}}{2^{2n}}
frac{(2n+q)!}{(n-q/2)! times (n+q/2)!}.}$$
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Thanks very much, I really appreciate and start to like that coefficient extractor method ;)
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– Diger
Dec 25 '18 at 22:26
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@Diger Thank you for the kind remark. You might want to add some context if it was useful to you, so it gets re-opened.
$endgroup$
– Marko Riedel
Dec 29 '18 at 18:51
add a comment |
1 Answer
1
active
oldest
votes
1 Answer
1
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
We seek to evaluate
$$sum_{k=q}^{2n} {2n+kchoose 2k}
frac{(2k-1)!}{(k-1)! times 2^{k-1}} frac{1}{(k-q)!} (-1)^k
\ = sum_{k=q}^{2n} {2n+kchoose 2k}
frac{(2k)!}{k! times 2^{k}} frac{1}{(k-q)!} (-1)^k
.$$
This is
$$q! sum_{k=q}^{2n} {2n+kchoose 2k}
frac{(2k)!}{k! times k! times 2^{k}}
frac{k!}{q!times(k-q)!} frac{(-1)^k}{2^k}
\ = q! sum_{k=q}^{2n} {2n+kchoose 2k}
{2kchoose k} {kchoose q}
frac{(-1)^k}{2^k}.$$
Observe that
$${2n+kchoose 2k} {2kchoose k}
= frac{(2n+k)!}{(2n-k)! times k! times k!}
= {2n+kchoose 2n} {2nchoose k}$$
and furthermore
$${2nchoose k} {kchoose q}
= frac{(2n)!}{(2n-k)! times q! times (k-q)!}
= {2nchoose q} {2n-qchoose k-q}.$$
We get for the sum
$${2nchoose q} q!
sum_{k=q}^{2n} {2n+kchoose 2n} {2n-qchoose k-q}
frac{(-1)^k}{2^k}
\ = {2nchoose q} q! frac{(-1)^q}{2^q}
sum_{k=0}^{2n-q} {2n+q+kchoose 2n} {2n-qchoose k}
frac{(-1)^k}{2^k}
.$$
This becomes
$${2nchoose q} q! frac{(-1)^q}{2^q}
sum_{k=0}^{2n-q} {2n+q+kchoose 2n} [z^{2n-q-k}] (1+z)^{2n-q}
frac{(-1)^k}{2^k}
\ = {2nchoose q} q! frac{(-1)^q}{2^q}
[z^{2n-q}] (1+z)^{2n-q}
sum_{k=0}^{2n-q} {2n+q+kchoose 2n}
frac{(-1)^k}{2^k} z^k.$$
Now we may extend $k$ beyond $2n-q$ because of the coefficient
extractor $[z^{2n-q}]$ (no contribution) and get
$${2nchoose q} q! frac{(-1)^q}{2^q}
[z^{2n-q}] (1+z)^{2n-q}
sum_{kge 0} {2n+q+kchoose 2n}
frac{(-1)^k}{2^k} z^k
\ = {2nchoose q} q! frac{(-1)^q}{2^q}
[z^{2n-q}] (1+z)^{2n-q} [w^{2n}] (1+w)^{2n+q}
sum_{kge 0} (1+w)^k
frac{(-1)^k}{2^k} z^k
\ = {2nchoose q} q! frac{(-1)^q}{2^q}
[z^{2n-q}] (1+z)^{2n-q} [w^{2n}] (1+w)^{2n+q}
frac{1}{1+z(1+w)/2}.$$
Re-write this as
$${2nchoose q} q! frac{(-1)^q}{2^q}
[w^{2n}] (1+w)^{2n+q}
mathrm{Res}_{z=0} frac{1}{z^{2n-q+1}} (1+z)^{2n-q}
frac{1}{1+z(1+w)/2}.$$
Working with the residue we apply the substitution $z/(1+z) = v$
or $z=v/(1-v)$ to get
$$mathrm{Res}_{v=0} frac{1}{v^{2n-q}}
frac{1-v}{v} frac{1}{1+(v/(1-v))(1+w)/2} frac{1}{(1-v)^2}
\ = mathrm{Res}_{v=0} frac{1}{v^{2n-q+1}}
frac{1}{1-v+v(1+w)/2}
\ = mathrm{Res}_{v=0} frac{1}{v^{2n-q+1}}
frac{1}{1-v(1-w)/2}
= frac{1}{2^{2n-q}} (1-w)^{2n-q}.$$
Substitute into the remaining coefficient extractor to get
$${2nchoose q} q! frac{(-1)^q}{2^q}
[w^{2n}] (1+w)^{2n+q} frac{1}{2^{2n-q}} (1-w)^{2n-q}
\ = {2nchoose q} q! frac{(-1)^q}{2^{2n}}
sum_{p=0}^{2n-q} (-1)^p {2n-qchoose p} {2n+qchoose 2n-p}.$$
Now
$${2nchoose q} {2n-qchoose p}
= frac{(2n)!}{q!times p! times (2n-q-p)!}
= {2nchoose p} {2n-pchoose q}$$
and
$${2n-pchoose q} {2n+qchoose 2n-p}
= frac{(2n+q)!}{q! times (2n-p-q)! times (p+q)!}
= {2n+qchoose q} {2nchoose p+q}.$$
This yields
$${2n+qchoose q} q! frac{(-1)^q}{2^{2n}}
sum_{p=0}^{2n-q} (-1)^p {2nchoose p} {2nchoose p+q}
\ = {2n+qchoose q} q! frac{(-1)^q}{2^{2n}}
sum_{p=0}^{2n-q} (-1)^p {2nchoose p}
[z^{2n-p-q}] (1+z)^{2n}
\ = {2n+qchoose q} q! frac{(-1)^q}{2^{2n}}
[z^{2n-q}] (1+z)^{2n}
sum_{p=0}^{2n-q} (-1)^p {2nchoose p} z^p.$$
Now we may extend $p$ beyond $2n-q$ because of the coeffcient
extractor $[z^{2n-q}]$ in front. We find
$${2n+qchoose q} q! frac{(-1)^q}{2^{2n}}
[z^{2n-q}] (1+z)^{2n}
sum_{pge 0} (-1)^p {2nchoose p} z^p
\ = {2n+qchoose q} q! frac{(-1)^q}{2^{2n}}
[z^{2n-q}] (1+z)^{2n} (1-z)^{2n}
\ = {2n+qchoose q} q! frac{(-1)^q}{2^{2n}}
[z^{2n-q}] (1-z^2)^{2n}.$$
Concluding we immediately obtain zero when $q$ is odd, and otherwise
we find
$${2n+qchoose q} q! frac{(-1)^q}{2^{2n}}
[z^{2(n-q/2)}] (1-z^2)^{2n}
\ = {2n+qchoose q} q! frac{(-1)^q}{2^{2n}}
[z^{n-q/2}] (1-z)^{2n}.$$
This is
$${2n+qchoose q} q! frac{(-1)^q}{2^{2n}}
(-1)^{n-q/2} {2nchoose n-q/2}$$
or alternatively
$$bbox[5px,border:2px solid #00A000]{
frac{(-1)^{n+q/2}}{2^{2n}}
frac{(2n+q)!}{(n-q/2)! times (n+q/2)!}.}$$
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Thanks very much, I really appreciate and start to like that coefficient extractor method ;)
$endgroup$
– Diger
Dec 25 '18 at 22:26
$begingroup$
@Diger Thank you for the kind remark. You might want to add some context if it was useful to you, so it gets re-opened.
$endgroup$
– Marko Riedel
Dec 29 '18 at 18:51
add a comment |
$begingroup$
We seek to evaluate
$$sum_{k=q}^{2n} {2n+kchoose 2k}
frac{(2k-1)!}{(k-1)! times 2^{k-1}} frac{1}{(k-q)!} (-1)^k
\ = sum_{k=q}^{2n} {2n+kchoose 2k}
frac{(2k)!}{k! times 2^{k}} frac{1}{(k-q)!} (-1)^k
.$$
This is
$$q! sum_{k=q}^{2n} {2n+kchoose 2k}
frac{(2k)!}{k! times k! times 2^{k}}
frac{k!}{q!times(k-q)!} frac{(-1)^k}{2^k}
\ = q! sum_{k=q}^{2n} {2n+kchoose 2k}
{2kchoose k} {kchoose q}
frac{(-1)^k}{2^k}.$$
Observe that
$${2n+kchoose 2k} {2kchoose k}
= frac{(2n+k)!}{(2n-k)! times k! times k!}
= {2n+kchoose 2n} {2nchoose k}$$
and furthermore
$${2nchoose k} {kchoose q}
= frac{(2n)!}{(2n-k)! times q! times (k-q)!}
= {2nchoose q} {2n-qchoose k-q}.$$
We get for the sum
$${2nchoose q} q!
sum_{k=q}^{2n} {2n+kchoose 2n} {2n-qchoose k-q}
frac{(-1)^k}{2^k}
\ = {2nchoose q} q! frac{(-1)^q}{2^q}
sum_{k=0}^{2n-q} {2n+q+kchoose 2n} {2n-qchoose k}
frac{(-1)^k}{2^k}
.$$
This becomes
$${2nchoose q} q! frac{(-1)^q}{2^q}
sum_{k=0}^{2n-q} {2n+q+kchoose 2n} [z^{2n-q-k}] (1+z)^{2n-q}
frac{(-1)^k}{2^k}
\ = {2nchoose q} q! frac{(-1)^q}{2^q}
[z^{2n-q}] (1+z)^{2n-q}
sum_{k=0}^{2n-q} {2n+q+kchoose 2n}
frac{(-1)^k}{2^k} z^k.$$
Now we may extend $k$ beyond $2n-q$ because of the coefficient
extractor $[z^{2n-q}]$ (no contribution) and get
$${2nchoose q} q! frac{(-1)^q}{2^q}
[z^{2n-q}] (1+z)^{2n-q}
sum_{kge 0} {2n+q+kchoose 2n}
frac{(-1)^k}{2^k} z^k
\ = {2nchoose q} q! frac{(-1)^q}{2^q}
[z^{2n-q}] (1+z)^{2n-q} [w^{2n}] (1+w)^{2n+q}
sum_{kge 0} (1+w)^k
frac{(-1)^k}{2^k} z^k
\ = {2nchoose q} q! frac{(-1)^q}{2^q}
[z^{2n-q}] (1+z)^{2n-q} [w^{2n}] (1+w)^{2n+q}
frac{1}{1+z(1+w)/2}.$$
Re-write this as
$${2nchoose q} q! frac{(-1)^q}{2^q}
[w^{2n}] (1+w)^{2n+q}
mathrm{Res}_{z=0} frac{1}{z^{2n-q+1}} (1+z)^{2n-q}
frac{1}{1+z(1+w)/2}.$$
Working with the residue we apply the substitution $z/(1+z) = v$
or $z=v/(1-v)$ to get
$$mathrm{Res}_{v=0} frac{1}{v^{2n-q}}
frac{1-v}{v} frac{1}{1+(v/(1-v))(1+w)/2} frac{1}{(1-v)^2}
\ = mathrm{Res}_{v=0} frac{1}{v^{2n-q+1}}
frac{1}{1-v+v(1+w)/2}
\ = mathrm{Res}_{v=0} frac{1}{v^{2n-q+1}}
frac{1}{1-v(1-w)/2}
= frac{1}{2^{2n-q}} (1-w)^{2n-q}.$$
Substitute into the remaining coefficient extractor to get
$${2nchoose q} q! frac{(-1)^q}{2^q}
[w^{2n}] (1+w)^{2n+q} frac{1}{2^{2n-q}} (1-w)^{2n-q}
\ = {2nchoose q} q! frac{(-1)^q}{2^{2n}}
sum_{p=0}^{2n-q} (-1)^p {2n-qchoose p} {2n+qchoose 2n-p}.$$
Now
$${2nchoose q} {2n-qchoose p}
= frac{(2n)!}{q!times p! times (2n-q-p)!}
= {2nchoose p} {2n-pchoose q}$$
and
$${2n-pchoose q} {2n+qchoose 2n-p}
= frac{(2n+q)!}{q! times (2n-p-q)! times (p+q)!}
= {2n+qchoose q} {2nchoose p+q}.$$
This yields
$${2n+qchoose q} q! frac{(-1)^q}{2^{2n}}
sum_{p=0}^{2n-q} (-1)^p {2nchoose p} {2nchoose p+q}
\ = {2n+qchoose q} q! frac{(-1)^q}{2^{2n}}
sum_{p=0}^{2n-q} (-1)^p {2nchoose p}
[z^{2n-p-q}] (1+z)^{2n}
\ = {2n+qchoose q} q! frac{(-1)^q}{2^{2n}}
[z^{2n-q}] (1+z)^{2n}
sum_{p=0}^{2n-q} (-1)^p {2nchoose p} z^p.$$
Now we may extend $p$ beyond $2n-q$ because of the coeffcient
extractor $[z^{2n-q}]$ in front. We find
$${2n+qchoose q} q! frac{(-1)^q}{2^{2n}}
[z^{2n-q}] (1+z)^{2n}
sum_{pge 0} (-1)^p {2nchoose p} z^p
\ = {2n+qchoose q} q! frac{(-1)^q}{2^{2n}}
[z^{2n-q}] (1+z)^{2n} (1-z)^{2n}
\ = {2n+qchoose q} q! frac{(-1)^q}{2^{2n}}
[z^{2n-q}] (1-z^2)^{2n}.$$
Concluding we immediately obtain zero when $q$ is odd, and otherwise
we find
$${2n+qchoose q} q! frac{(-1)^q}{2^{2n}}
[z^{2(n-q/2)}] (1-z^2)^{2n}
\ = {2n+qchoose q} q! frac{(-1)^q}{2^{2n}}
[z^{n-q/2}] (1-z)^{2n}.$$
This is
$${2n+qchoose q} q! frac{(-1)^q}{2^{2n}}
(-1)^{n-q/2} {2nchoose n-q/2}$$
or alternatively
$$bbox[5px,border:2px solid #00A000]{
frac{(-1)^{n+q/2}}{2^{2n}}
frac{(2n+q)!}{(n-q/2)! times (n+q/2)!}.}$$
$endgroup$
$begingroup$
Thanks very much, I really appreciate and start to like that coefficient extractor method ;)
$endgroup$
– Diger
Dec 25 '18 at 22:26
$begingroup$
@Diger Thank you for the kind remark. You might want to add some context if it was useful to you, so it gets re-opened.
$endgroup$
– Marko Riedel
Dec 29 '18 at 18:51
add a comment |
$begingroup$
We seek to evaluate
$$sum_{k=q}^{2n} {2n+kchoose 2k}
frac{(2k-1)!}{(k-1)! times 2^{k-1}} frac{1}{(k-q)!} (-1)^k
\ = sum_{k=q}^{2n} {2n+kchoose 2k}
frac{(2k)!}{k! times 2^{k}} frac{1}{(k-q)!} (-1)^k
.$$
This is
$$q! sum_{k=q}^{2n} {2n+kchoose 2k}
frac{(2k)!}{k! times k! times 2^{k}}
frac{k!}{q!times(k-q)!} frac{(-1)^k}{2^k}
\ = q! sum_{k=q}^{2n} {2n+kchoose 2k}
{2kchoose k} {kchoose q}
frac{(-1)^k}{2^k}.$$
Observe that
$${2n+kchoose 2k} {2kchoose k}
= frac{(2n+k)!}{(2n-k)! times k! times k!}
= {2n+kchoose 2n} {2nchoose k}$$
and furthermore
$${2nchoose k} {kchoose q}
= frac{(2n)!}{(2n-k)! times q! times (k-q)!}
= {2nchoose q} {2n-qchoose k-q}.$$
We get for the sum
$${2nchoose q} q!
sum_{k=q}^{2n} {2n+kchoose 2n} {2n-qchoose k-q}
frac{(-1)^k}{2^k}
\ = {2nchoose q} q! frac{(-1)^q}{2^q}
sum_{k=0}^{2n-q} {2n+q+kchoose 2n} {2n-qchoose k}
frac{(-1)^k}{2^k}
.$$
This becomes
$${2nchoose q} q! frac{(-1)^q}{2^q}
sum_{k=0}^{2n-q} {2n+q+kchoose 2n} [z^{2n-q-k}] (1+z)^{2n-q}
frac{(-1)^k}{2^k}
\ = {2nchoose q} q! frac{(-1)^q}{2^q}
[z^{2n-q}] (1+z)^{2n-q}
sum_{k=0}^{2n-q} {2n+q+kchoose 2n}
frac{(-1)^k}{2^k} z^k.$$
Now we may extend $k$ beyond $2n-q$ because of the coefficient
extractor $[z^{2n-q}]$ (no contribution) and get
$${2nchoose q} q! frac{(-1)^q}{2^q}
[z^{2n-q}] (1+z)^{2n-q}
sum_{kge 0} {2n+q+kchoose 2n}
frac{(-1)^k}{2^k} z^k
\ = {2nchoose q} q! frac{(-1)^q}{2^q}
[z^{2n-q}] (1+z)^{2n-q} [w^{2n}] (1+w)^{2n+q}
sum_{kge 0} (1+w)^k
frac{(-1)^k}{2^k} z^k
\ = {2nchoose q} q! frac{(-1)^q}{2^q}
[z^{2n-q}] (1+z)^{2n-q} [w^{2n}] (1+w)^{2n+q}
frac{1}{1+z(1+w)/2}.$$
Re-write this as
$${2nchoose q} q! frac{(-1)^q}{2^q}
[w^{2n}] (1+w)^{2n+q}
mathrm{Res}_{z=0} frac{1}{z^{2n-q+1}} (1+z)^{2n-q}
frac{1}{1+z(1+w)/2}.$$
Working with the residue we apply the substitution $z/(1+z) = v$
or $z=v/(1-v)$ to get
$$mathrm{Res}_{v=0} frac{1}{v^{2n-q}}
frac{1-v}{v} frac{1}{1+(v/(1-v))(1+w)/2} frac{1}{(1-v)^2}
\ = mathrm{Res}_{v=0} frac{1}{v^{2n-q+1}}
frac{1}{1-v+v(1+w)/2}
\ = mathrm{Res}_{v=0} frac{1}{v^{2n-q+1}}
frac{1}{1-v(1-w)/2}
= frac{1}{2^{2n-q}} (1-w)^{2n-q}.$$
Substitute into the remaining coefficient extractor to get
$${2nchoose q} q! frac{(-1)^q}{2^q}
[w^{2n}] (1+w)^{2n+q} frac{1}{2^{2n-q}} (1-w)^{2n-q}
\ = {2nchoose q} q! frac{(-1)^q}{2^{2n}}
sum_{p=0}^{2n-q} (-1)^p {2n-qchoose p} {2n+qchoose 2n-p}.$$
Now
$${2nchoose q} {2n-qchoose p}
= frac{(2n)!}{q!times p! times (2n-q-p)!}
= {2nchoose p} {2n-pchoose q}$$
and
$${2n-pchoose q} {2n+qchoose 2n-p}
= frac{(2n+q)!}{q! times (2n-p-q)! times (p+q)!}
= {2n+qchoose q} {2nchoose p+q}.$$
This yields
$${2n+qchoose q} q! frac{(-1)^q}{2^{2n}}
sum_{p=0}^{2n-q} (-1)^p {2nchoose p} {2nchoose p+q}
\ = {2n+qchoose q} q! frac{(-1)^q}{2^{2n}}
sum_{p=0}^{2n-q} (-1)^p {2nchoose p}
[z^{2n-p-q}] (1+z)^{2n}
\ = {2n+qchoose q} q! frac{(-1)^q}{2^{2n}}
[z^{2n-q}] (1+z)^{2n}
sum_{p=0}^{2n-q} (-1)^p {2nchoose p} z^p.$$
Now we may extend $p$ beyond $2n-q$ because of the coeffcient
extractor $[z^{2n-q}]$ in front. We find
$${2n+qchoose q} q! frac{(-1)^q}{2^{2n}}
[z^{2n-q}] (1+z)^{2n}
sum_{pge 0} (-1)^p {2nchoose p} z^p
\ = {2n+qchoose q} q! frac{(-1)^q}{2^{2n}}
[z^{2n-q}] (1+z)^{2n} (1-z)^{2n}
\ = {2n+qchoose q} q! frac{(-1)^q}{2^{2n}}
[z^{2n-q}] (1-z^2)^{2n}.$$
Concluding we immediately obtain zero when $q$ is odd, and otherwise
we find
$${2n+qchoose q} q! frac{(-1)^q}{2^{2n}}
[z^{2(n-q/2)}] (1-z^2)^{2n}
\ = {2n+qchoose q} q! frac{(-1)^q}{2^{2n}}
[z^{n-q/2}] (1-z)^{2n}.$$
This is
$${2n+qchoose q} q! frac{(-1)^q}{2^{2n}}
(-1)^{n-q/2} {2nchoose n-q/2}$$
or alternatively
$$bbox[5px,border:2px solid #00A000]{
frac{(-1)^{n+q/2}}{2^{2n}}
frac{(2n+q)!}{(n-q/2)! times (n+q/2)!}.}$$
$endgroup$
We seek to evaluate
$$sum_{k=q}^{2n} {2n+kchoose 2k}
frac{(2k-1)!}{(k-1)! times 2^{k-1}} frac{1}{(k-q)!} (-1)^k
\ = sum_{k=q}^{2n} {2n+kchoose 2k}
frac{(2k)!}{k! times 2^{k}} frac{1}{(k-q)!} (-1)^k
.$$
This is
$$q! sum_{k=q}^{2n} {2n+kchoose 2k}
frac{(2k)!}{k! times k! times 2^{k}}
frac{k!}{q!times(k-q)!} frac{(-1)^k}{2^k}
\ = q! sum_{k=q}^{2n} {2n+kchoose 2k}
{2kchoose k} {kchoose q}
frac{(-1)^k}{2^k}.$$
Observe that
$${2n+kchoose 2k} {2kchoose k}
= frac{(2n+k)!}{(2n-k)! times k! times k!}
= {2n+kchoose 2n} {2nchoose k}$$
and furthermore
$${2nchoose k} {kchoose q}
= frac{(2n)!}{(2n-k)! times q! times (k-q)!}
= {2nchoose q} {2n-qchoose k-q}.$$
We get for the sum
$${2nchoose q} q!
sum_{k=q}^{2n} {2n+kchoose 2n} {2n-qchoose k-q}
frac{(-1)^k}{2^k}
\ = {2nchoose q} q! frac{(-1)^q}{2^q}
sum_{k=0}^{2n-q} {2n+q+kchoose 2n} {2n-qchoose k}
frac{(-1)^k}{2^k}
.$$
This becomes
$${2nchoose q} q! frac{(-1)^q}{2^q}
sum_{k=0}^{2n-q} {2n+q+kchoose 2n} [z^{2n-q-k}] (1+z)^{2n-q}
frac{(-1)^k}{2^k}
\ = {2nchoose q} q! frac{(-1)^q}{2^q}
[z^{2n-q}] (1+z)^{2n-q}
sum_{k=0}^{2n-q} {2n+q+kchoose 2n}
frac{(-1)^k}{2^k} z^k.$$
Now we may extend $k$ beyond $2n-q$ because of the coefficient
extractor $[z^{2n-q}]$ (no contribution) and get
$${2nchoose q} q! frac{(-1)^q}{2^q}
[z^{2n-q}] (1+z)^{2n-q}
sum_{kge 0} {2n+q+kchoose 2n}
frac{(-1)^k}{2^k} z^k
\ = {2nchoose q} q! frac{(-1)^q}{2^q}
[z^{2n-q}] (1+z)^{2n-q} [w^{2n}] (1+w)^{2n+q}
sum_{kge 0} (1+w)^k
frac{(-1)^k}{2^k} z^k
\ = {2nchoose q} q! frac{(-1)^q}{2^q}
[z^{2n-q}] (1+z)^{2n-q} [w^{2n}] (1+w)^{2n+q}
frac{1}{1+z(1+w)/2}.$$
Re-write this as
$${2nchoose q} q! frac{(-1)^q}{2^q}
[w^{2n}] (1+w)^{2n+q}
mathrm{Res}_{z=0} frac{1}{z^{2n-q+1}} (1+z)^{2n-q}
frac{1}{1+z(1+w)/2}.$$
Working with the residue we apply the substitution $z/(1+z) = v$
or $z=v/(1-v)$ to get
$$mathrm{Res}_{v=0} frac{1}{v^{2n-q}}
frac{1-v}{v} frac{1}{1+(v/(1-v))(1+w)/2} frac{1}{(1-v)^2}
\ = mathrm{Res}_{v=0} frac{1}{v^{2n-q+1}}
frac{1}{1-v+v(1+w)/2}
\ = mathrm{Res}_{v=0} frac{1}{v^{2n-q+1}}
frac{1}{1-v(1-w)/2}
= frac{1}{2^{2n-q}} (1-w)^{2n-q}.$$
Substitute into the remaining coefficient extractor to get
$${2nchoose q} q! frac{(-1)^q}{2^q}
[w^{2n}] (1+w)^{2n+q} frac{1}{2^{2n-q}} (1-w)^{2n-q}
\ = {2nchoose q} q! frac{(-1)^q}{2^{2n}}
sum_{p=0}^{2n-q} (-1)^p {2n-qchoose p} {2n+qchoose 2n-p}.$$
Now
$${2nchoose q} {2n-qchoose p}
= frac{(2n)!}{q!times p! times (2n-q-p)!}
= {2nchoose p} {2n-pchoose q}$$
and
$${2n-pchoose q} {2n+qchoose 2n-p}
= frac{(2n+q)!}{q! times (2n-p-q)! times (p+q)!}
= {2n+qchoose q} {2nchoose p+q}.$$
This yields
$${2n+qchoose q} q! frac{(-1)^q}{2^{2n}}
sum_{p=0}^{2n-q} (-1)^p {2nchoose p} {2nchoose p+q}
\ = {2n+qchoose q} q! frac{(-1)^q}{2^{2n}}
sum_{p=0}^{2n-q} (-1)^p {2nchoose p}
[z^{2n-p-q}] (1+z)^{2n}
\ = {2n+qchoose q} q! frac{(-1)^q}{2^{2n}}
[z^{2n-q}] (1+z)^{2n}
sum_{p=0}^{2n-q} (-1)^p {2nchoose p} z^p.$$
Now we may extend $p$ beyond $2n-q$ because of the coeffcient
extractor $[z^{2n-q}]$ in front. We find
$${2n+qchoose q} q! frac{(-1)^q}{2^{2n}}
[z^{2n-q}] (1+z)^{2n}
sum_{pge 0} (-1)^p {2nchoose p} z^p
\ = {2n+qchoose q} q! frac{(-1)^q}{2^{2n}}
[z^{2n-q}] (1+z)^{2n} (1-z)^{2n}
\ = {2n+qchoose q} q! frac{(-1)^q}{2^{2n}}
[z^{2n-q}] (1-z^2)^{2n}.$$
Concluding we immediately obtain zero when $q$ is odd, and otherwise
we find
$${2n+qchoose q} q! frac{(-1)^q}{2^{2n}}
[z^{2(n-q/2)}] (1-z^2)^{2n}
\ = {2n+qchoose q} q! frac{(-1)^q}{2^{2n}}
[z^{n-q/2}] (1-z)^{2n}.$$
This is
$${2n+qchoose q} q! frac{(-1)^q}{2^{2n}}
(-1)^{n-q/2} {2nchoose n-q/2}$$
or alternatively
$$bbox[5px,border:2px solid #00A000]{
frac{(-1)^{n+q/2}}{2^{2n}}
frac{(2n+q)!}{(n-q/2)! times (n+q/2)!}.}$$
answered Dec 25 '18 at 21:40
Marko RiedelMarko Riedel
40.4k339109
40.4k339109
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Thanks very much, I really appreciate and start to like that coefficient extractor method ;)
$endgroup$
– Diger
Dec 25 '18 at 22:26
$begingroup$
@Diger Thank you for the kind remark. You might want to add some context if it was useful to you, so it gets re-opened.
$endgroup$
– Marko Riedel
Dec 29 '18 at 18:51
add a comment |
$begingroup$
Thanks very much, I really appreciate and start to like that coefficient extractor method ;)
$endgroup$
– Diger
Dec 25 '18 at 22:26
$begingroup$
@Diger Thank you for the kind remark. You might want to add some context if it was useful to you, so it gets re-opened.
$endgroup$
– Marko Riedel
Dec 29 '18 at 18:51
$begingroup$
Thanks very much, I really appreciate and start to like that coefficient extractor method ;)
$endgroup$
– Diger
Dec 25 '18 at 22:26
$begingroup$
Thanks very much, I really appreciate and start to like that coefficient extractor method ;)
$endgroup$
– Diger
Dec 25 '18 at 22:26
$begingroup$
@Diger Thank you for the kind remark. You might want to add some context if it was useful to you, so it gets re-opened.
$endgroup$
– Marko Riedel
Dec 29 '18 at 18:51
$begingroup$
@Diger Thank you for the kind remark. You might want to add some context if it was useful to you, so it gets re-opened.
$endgroup$
– Marko Riedel
Dec 29 '18 at 18:51
add a comment |
2
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I have not down-voted, but perhaps is because you haven't include any information, like how did you encounter this problem? Or how did you obtain the answer?
$endgroup$
– Zacky
Dec 25 '18 at 21:43