Convergency of $sum_{n=0}^{infty}frac{n!}{(kn)!}$, where $k > 1$
$begingroup$
I am confident that $$sum_{n=1}^{infty}frac{n!}{(2n)!}approx1.5923$$
converges. Other series such as
$$sum_{n=1}^{infty}frac{n!}{(1.1n)!}approx5.5690$$
appear to converge as well, and
my hypothesis is that the series
$$sum_{n=0}^{infty}frac{n!}{(kn)!}, k > 1$$
converges.
I know that the $p$-series converges for $p>1$ due to the following relationship:
$$int_{1}^{infty}frac{1}{x^p}dx < sum_{1}^{infty}frac{1}{x^p}<1+int_{1}^{infty}frac{1}{x^p}dx$$
However, I am not sure how to prove that
$$sum_{n=0}^{infty}frac{n!}{(kn)!}, k > 1$$
converges with the same method that is used to prove the convergency of the $p$-series.
Could you provide me some hints?
calculus sequences-and-series
$endgroup$
|
show 2 more comments
$begingroup$
I am confident that $$sum_{n=1}^{infty}frac{n!}{(2n)!}approx1.5923$$
converges. Other series such as
$$sum_{n=1}^{infty}frac{n!}{(1.1n)!}approx5.5690$$
appear to converge as well, and
my hypothesis is that the series
$$sum_{n=0}^{infty}frac{n!}{(kn)!}, k > 1$$
converges.
I know that the $p$-series converges for $p>1$ due to the following relationship:
$$int_{1}^{infty}frac{1}{x^p}dx < sum_{1}^{infty}frac{1}{x^p}<1+int_{1}^{infty}frac{1}{x^p}dx$$
However, I am not sure how to prove that
$$sum_{n=0}^{infty}frac{n!}{(kn)!}, k > 1$$
converges with the same method that is used to prove the convergency of the $p$-series.
Could you provide me some hints?
calculus sequences-and-series
$endgroup$
1
$begingroup$
Have you tried ratio test?
$endgroup$
– Anurag A
Oct 2 '18 at 21:47
$begingroup$
What is $(1.1n)!$?
$endgroup$
– Hagen von Eitzen
Oct 2 '18 at 21:59
1
$begingroup$
Note that $frac{n!}{(kn)!}=frac{1}{{knchoose n}(kn-n)!}<frac1{n!}$ for $kge 2$
$endgroup$
– Hagen von Eitzen
Oct 2 '18 at 22:00
$begingroup$
@Hagen It is a coefficient. 11/10
$endgroup$
– Larry
Oct 2 '18 at 22:01
$begingroup$
@Larry: then what is $1.1! $ ?
$endgroup$
– Yves Daoust
Oct 2 '18 at 22:06
|
show 2 more comments
$begingroup$
I am confident that $$sum_{n=1}^{infty}frac{n!}{(2n)!}approx1.5923$$
converges. Other series such as
$$sum_{n=1}^{infty}frac{n!}{(1.1n)!}approx5.5690$$
appear to converge as well, and
my hypothesis is that the series
$$sum_{n=0}^{infty}frac{n!}{(kn)!}, k > 1$$
converges.
I know that the $p$-series converges for $p>1$ due to the following relationship:
$$int_{1}^{infty}frac{1}{x^p}dx < sum_{1}^{infty}frac{1}{x^p}<1+int_{1}^{infty}frac{1}{x^p}dx$$
However, I am not sure how to prove that
$$sum_{n=0}^{infty}frac{n!}{(kn)!}, k > 1$$
converges with the same method that is used to prove the convergency of the $p$-series.
Could you provide me some hints?
calculus sequences-and-series
$endgroup$
I am confident that $$sum_{n=1}^{infty}frac{n!}{(2n)!}approx1.5923$$
converges. Other series such as
$$sum_{n=1}^{infty}frac{n!}{(1.1n)!}approx5.5690$$
appear to converge as well, and
my hypothesis is that the series
$$sum_{n=0}^{infty}frac{n!}{(kn)!}, k > 1$$
converges.
I know that the $p$-series converges for $p>1$ due to the following relationship:
$$int_{1}^{infty}frac{1}{x^p}dx < sum_{1}^{infty}frac{1}{x^p}<1+int_{1}^{infty}frac{1}{x^p}dx$$
However, I am not sure how to prove that
$$sum_{n=0}^{infty}frac{n!}{(kn)!}, k > 1$$
converges with the same method that is used to prove the convergency of the $p$-series.
Could you provide me some hints?
calculus sequences-and-series
calculus sequences-and-series
asked Oct 2 '18 at 21:45
LarryLarry
2,40831129
2,40831129
1
$begingroup$
Have you tried ratio test?
$endgroup$
– Anurag A
Oct 2 '18 at 21:47
$begingroup$
What is $(1.1n)!$?
$endgroup$
– Hagen von Eitzen
Oct 2 '18 at 21:59
1
$begingroup$
Note that $frac{n!}{(kn)!}=frac{1}{{knchoose n}(kn-n)!}<frac1{n!}$ for $kge 2$
$endgroup$
– Hagen von Eitzen
Oct 2 '18 at 22:00
$begingroup$
@Hagen It is a coefficient. 11/10
$endgroup$
– Larry
Oct 2 '18 at 22:01
$begingroup$
@Larry: then what is $1.1! $ ?
$endgroup$
– Yves Daoust
Oct 2 '18 at 22:06
|
show 2 more comments
1
$begingroup$
Have you tried ratio test?
$endgroup$
– Anurag A
Oct 2 '18 at 21:47
$begingroup$
What is $(1.1n)!$?
$endgroup$
– Hagen von Eitzen
Oct 2 '18 at 21:59
1
$begingroup$
Note that $frac{n!}{(kn)!}=frac{1}{{knchoose n}(kn-n)!}<frac1{n!}$ for $kge 2$
$endgroup$
– Hagen von Eitzen
Oct 2 '18 at 22:00
$begingroup$
@Hagen It is a coefficient. 11/10
$endgroup$
– Larry
Oct 2 '18 at 22:01
$begingroup$
@Larry: then what is $1.1! $ ?
$endgroup$
– Yves Daoust
Oct 2 '18 at 22:06
1
1
$begingroup$
Have you tried ratio test?
$endgroup$
– Anurag A
Oct 2 '18 at 21:47
$begingroup$
Have you tried ratio test?
$endgroup$
– Anurag A
Oct 2 '18 at 21:47
$begingroup$
What is $(1.1n)!$?
$endgroup$
– Hagen von Eitzen
Oct 2 '18 at 21:59
$begingroup$
What is $(1.1n)!$?
$endgroup$
– Hagen von Eitzen
Oct 2 '18 at 21:59
1
1
$begingroup$
Note that $frac{n!}{(kn)!}=frac{1}{{knchoose n}(kn-n)!}<frac1{n!}$ for $kge 2$
$endgroup$
– Hagen von Eitzen
Oct 2 '18 at 22:00
$begingroup$
Note that $frac{n!}{(kn)!}=frac{1}{{knchoose n}(kn-n)!}<frac1{n!}$ for $kge 2$
$endgroup$
– Hagen von Eitzen
Oct 2 '18 at 22:00
$begingroup$
@Hagen It is a coefficient. 11/10
$endgroup$
– Larry
Oct 2 '18 at 22:01
$begingroup$
@Hagen It is a coefficient. 11/10
$endgroup$
– Larry
Oct 2 '18 at 22:01
$begingroup$
@Larry: then what is $1.1! $ ?
$endgroup$
– Yves Daoust
Oct 2 '18 at 22:06
$begingroup$
@Larry: then what is $1.1! $ ?
$endgroup$
– Yves Daoust
Oct 2 '18 at 22:06
|
show 2 more comments
3 Answers
3
active
oldest
votes
$begingroup$
$logGamma(s+1)$ is a convex function on $mathbb{R}^+$ and
$$ frac{n!}{(kn)!} = frac{Gamma(n+1)}{Gamma(kn+1)}leq frac{1}{n^{(k-1)n}}.$$
Something similar but more accurate can be deduced from Stirling's inequality
$$ left(frac{m}{e}right)^msqrt{2pi m}, e^{frac{1}{12m+1}}leqGamma(m+1) leq left(frac{m}{e}right)^msqrt{2pi m}, e^{frac{1}{12m}}.$$
$endgroup$
add a comment |
$begingroup$
By elementary means:
For any natural $m$,
$$kn-nge m$$ for sufficiently large $n$, so that $$dfrac{(kn)!}{n!}gelfloor knrfloorlfloor kn-1rfloorcdots(n+1)>n^m.$$
Hence the series converges.
$endgroup$
$begingroup$
I like the direction your answer is taking, but could you clarify how $m$ appears in the product you have written out? Maybe with underbrace?
$endgroup$
– AOrtiz
Oct 2 '18 at 22:48
1
$begingroup$
@AOrtiz: the number of factors in the product is the difference between the largest and the smallest, $lfloor knrfloor-n$.
$endgroup$
– Yves Daoust
Oct 2 '18 at 22:49
$begingroup$
Nice, thanks for the clarification! +1
$endgroup$
– AOrtiz
Oct 2 '18 at 22:50
add a comment |
$begingroup$
Since $k > 1$, write $k > 1 + frac{1}{N}$ for some $N$ sufficiently large. Then if $n > 2N$, we have $$kn > big(1+frac{1}{N}big)n = n + frac{n}{N} > n+2.$$
Thus for such $N$, we have
$$sum_{n> 2N} frac{n!}{(kn)!} le sum_{n>2N} frac{n!}{(n+2)!} =sum_{n>2N}frac{1}{(n+2)(n+1)} le sum_{n>2N}frac{1}{n^2},$$ which converges by the $p$-test.
$endgroup$
add a comment |
Your Answer
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3 Answers
3
active
oldest
votes
3 Answers
3
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
$logGamma(s+1)$ is a convex function on $mathbb{R}^+$ and
$$ frac{n!}{(kn)!} = frac{Gamma(n+1)}{Gamma(kn+1)}leq frac{1}{n^{(k-1)n}}.$$
Something similar but more accurate can be deduced from Stirling's inequality
$$ left(frac{m}{e}right)^msqrt{2pi m}, e^{frac{1}{12m+1}}leqGamma(m+1) leq left(frac{m}{e}right)^msqrt{2pi m}, e^{frac{1}{12m}}.$$
$endgroup$
add a comment |
$begingroup$
$logGamma(s+1)$ is a convex function on $mathbb{R}^+$ and
$$ frac{n!}{(kn)!} = frac{Gamma(n+1)}{Gamma(kn+1)}leq frac{1}{n^{(k-1)n}}.$$
Something similar but more accurate can be deduced from Stirling's inequality
$$ left(frac{m}{e}right)^msqrt{2pi m}, e^{frac{1}{12m+1}}leqGamma(m+1) leq left(frac{m}{e}right)^msqrt{2pi m}, e^{frac{1}{12m}}.$$
$endgroup$
add a comment |
$begingroup$
$logGamma(s+1)$ is a convex function on $mathbb{R}^+$ and
$$ frac{n!}{(kn)!} = frac{Gamma(n+1)}{Gamma(kn+1)}leq frac{1}{n^{(k-1)n}}.$$
Something similar but more accurate can be deduced from Stirling's inequality
$$ left(frac{m}{e}right)^msqrt{2pi m}, e^{frac{1}{12m+1}}leqGamma(m+1) leq left(frac{m}{e}right)^msqrt{2pi m}, e^{frac{1}{12m}}.$$
$endgroup$
$logGamma(s+1)$ is a convex function on $mathbb{R}^+$ and
$$ frac{n!}{(kn)!} = frac{Gamma(n+1)}{Gamma(kn+1)}leq frac{1}{n^{(k-1)n}}.$$
Something similar but more accurate can be deduced from Stirling's inequality
$$ left(frac{m}{e}right)^msqrt{2pi m}, e^{frac{1}{12m+1}}leqGamma(m+1) leq left(frac{m}{e}right)^msqrt{2pi m}, e^{frac{1}{12m}}.$$
edited Oct 2 '18 at 22:31
answered Oct 2 '18 at 22:14
Jack D'AurizioJack D'Aurizio
289k33281661
289k33281661
add a comment |
add a comment |
$begingroup$
By elementary means:
For any natural $m$,
$$kn-nge m$$ for sufficiently large $n$, so that $$dfrac{(kn)!}{n!}gelfloor knrfloorlfloor kn-1rfloorcdots(n+1)>n^m.$$
Hence the series converges.
$endgroup$
$begingroup$
I like the direction your answer is taking, but could you clarify how $m$ appears in the product you have written out? Maybe with underbrace?
$endgroup$
– AOrtiz
Oct 2 '18 at 22:48
1
$begingroup$
@AOrtiz: the number of factors in the product is the difference between the largest and the smallest, $lfloor knrfloor-n$.
$endgroup$
– Yves Daoust
Oct 2 '18 at 22:49
$begingroup$
Nice, thanks for the clarification! +1
$endgroup$
– AOrtiz
Oct 2 '18 at 22:50
add a comment |
$begingroup$
By elementary means:
For any natural $m$,
$$kn-nge m$$ for sufficiently large $n$, so that $$dfrac{(kn)!}{n!}gelfloor knrfloorlfloor kn-1rfloorcdots(n+1)>n^m.$$
Hence the series converges.
$endgroup$
$begingroup$
I like the direction your answer is taking, but could you clarify how $m$ appears in the product you have written out? Maybe with underbrace?
$endgroup$
– AOrtiz
Oct 2 '18 at 22:48
1
$begingroup$
@AOrtiz: the number of factors in the product is the difference between the largest and the smallest, $lfloor knrfloor-n$.
$endgroup$
– Yves Daoust
Oct 2 '18 at 22:49
$begingroup$
Nice, thanks for the clarification! +1
$endgroup$
– AOrtiz
Oct 2 '18 at 22:50
add a comment |
$begingroup$
By elementary means:
For any natural $m$,
$$kn-nge m$$ for sufficiently large $n$, so that $$dfrac{(kn)!}{n!}gelfloor knrfloorlfloor kn-1rfloorcdots(n+1)>n^m.$$
Hence the series converges.
$endgroup$
By elementary means:
For any natural $m$,
$$kn-nge m$$ for sufficiently large $n$, so that $$dfrac{(kn)!}{n!}gelfloor knrfloorlfloor kn-1rfloorcdots(n+1)>n^m.$$
Hence the series converges.
edited Oct 2 '18 at 22:43
answered Oct 2 '18 at 22:25
Yves DaoustYves Daoust
127k673226
127k673226
$begingroup$
I like the direction your answer is taking, but could you clarify how $m$ appears in the product you have written out? Maybe with underbrace?
$endgroup$
– AOrtiz
Oct 2 '18 at 22:48
1
$begingroup$
@AOrtiz: the number of factors in the product is the difference between the largest and the smallest, $lfloor knrfloor-n$.
$endgroup$
– Yves Daoust
Oct 2 '18 at 22:49
$begingroup$
Nice, thanks for the clarification! +1
$endgroup$
– AOrtiz
Oct 2 '18 at 22:50
add a comment |
$begingroup$
I like the direction your answer is taking, but could you clarify how $m$ appears in the product you have written out? Maybe with underbrace?
$endgroup$
– AOrtiz
Oct 2 '18 at 22:48
1
$begingroup$
@AOrtiz: the number of factors in the product is the difference between the largest and the smallest, $lfloor knrfloor-n$.
$endgroup$
– Yves Daoust
Oct 2 '18 at 22:49
$begingroup$
Nice, thanks for the clarification! +1
$endgroup$
– AOrtiz
Oct 2 '18 at 22:50
$begingroup$
I like the direction your answer is taking, but could you clarify how $m$ appears in the product you have written out? Maybe with underbrace?
$endgroup$
– AOrtiz
Oct 2 '18 at 22:48
$begingroup$
I like the direction your answer is taking, but could you clarify how $m$ appears in the product you have written out? Maybe with underbrace?
$endgroup$
– AOrtiz
Oct 2 '18 at 22:48
1
1
$begingroup$
@AOrtiz: the number of factors in the product is the difference between the largest and the smallest, $lfloor knrfloor-n$.
$endgroup$
– Yves Daoust
Oct 2 '18 at 22:49
$begingroup$
@AOrtiz: the number of factors in the product is the difference between the largest and the smallest, $lfloor knrfloor-n$.
$endgroup$
– Yves Daoust
Oct 2 '18 at 22:49
$begingroup$
Nice, thanks for the clarification! +1
$endgroup$
– AOrtiz
Oct 2 '18 at 22:50
$begingroup$
Nice, thanks for the clarification! +1
$endgroup$
– AOrtiz
Oct 2 '18 at 22:50
add a comment |
$begingroup$
Since $k > 1$, write $k > 1 + frac{1}{N}$ for some $N$ sufficiently large. Then if $n > 2N$, we have $$kn > big(1+frac{1}{N}big)n = n + frac{n}{N} > n+2.$$
Thus for such $N$, we have
$$sum_{n> 2N} frac{n!}{(kn)!} le sum_{n>2N} frac{n!}{(n+2)!} =sum_{n>2N}frac{1}{(n+2)(n+1)} le sum_{n>2N}frac{1}{n^2},$$ which converges by the $p$-test.
$endgroup$
add a comment |
$begingroup$
Since $k > 1$, write $k > 1 + frac{1}{N}$ for some $N$ sufficiently large. Then if $n > 2N$, we have $$kn > big(1+frac{1}{N}big)n = n + frac{n}{N} > n+2.$$
Thus for such $N$, we have
$$sum_{n> 2N} frac{n!}{(kn)!} le sum_{n>2N} frac{n!}{(n+2)!} =sum_{n>2N}frac{1}{(n+2)(n+1)} le sum_{n>2N}frac{1}{n^2},$$ which converges by the $p$-test.
$endgroup$
add a comment |
$begingroup$
Since $k > 1$, write $k > 1 + frac{1}{N}$ for some $N$ sufficiently large. Then if $n > 2N$, we have $$kn > big(1+frac{1}{N}big)n = n + frac{n}{N} > n+2.$$
Thus for such $N$, we have
$$sum_{n> 2N} frac{n!}{(kn)!} le sum_{n>2N} frac{n!}{(n+2)!} =sum_{n>2N}frac{1}{(n+2)(n+1)} le sum_{n>2N}frac{1}{n^2},$$ which converges by the $p$-test.
$endgroup$
Since $k > 1$, write $k > 1 + frac{1}{N}$ for some $N$ sufficiently large. Then if $n > 2N$, we have $$kn > big(1+frac{1}{N}big)n = n + frac{n}{N} > n+2.$$
Thus for such $N$, we have
$$sum_{n> 2N} frac{n!}{(kn)!} le sum_{n>2N} frac{n!}{(n+2)!} =sum_{n>2N}frac{1}{(n+2)(n+1)} le sum_{n>2N}frac{1}{n^2},$$ which converges by the $p$-test.
edited Jan 15 at 20:45
answered Oct 2 '18 at 22:26
AOrtizAOrtiz
10.6k21441
10.6k21441
add a comment |
add a comment |
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1
$begingroup$
Have you tried ratio test?
$endgroup$
– Anurag A
Oct 2 '18 at 21:47
$begingroup$
What is $(1.1n)!$?
$endgroup$
– Hagen von Eitzen
Oct 2 '18 at 21:59
1
$begingroup$
Note that $frac{n!}{(kn)!}=frac{1}{{knchoose n}(kn-n)!}<frac1{n!}$ for $kge 2$
$endgroup$
– Hagen von Eitzen
Oct 2 '18 at 22:00
$begingroup$
@Hagen It is a coefficient. 11/10
$endgroup$
– Larry
Oct 2 '18 at 22:01
$begingroup$
@Larry: then what is $1.1! $ ?
$endgroup$
– Yves Daoust
Oct 2 '18 at 22:06