For the periodic sequence, is there always an algebraic closed form?
$begingroup$
This question is a generalized form of the problem I asked before:
Algebraic Closed Form for $sum_{n=1}^{k}left( n- 3 lfloor frac{n-1}{3} rfloorright)$
Let, look at this periodic sequence:
$$a_n=left{a_1,a_2,a_3,a_4,a_5,cdots a_k ; a_1,a_2,a_3,a_4,a_5, cdots a_k; a_1,a_2,a_3,a_4,a_5,cdots a_k;cdots right}$$, where $left{a_1, a_2, a_3, cdots a_k right} in mathbb{Z^{+}}$ and $a_{k+1}=a_1, a_{k+2}=a_2, a_{k+3}=a_3, cdots a_{2k}=a_k, cdots$
For the sequence $a_k=left{ a_1,a_2,a_3,...,a_k right}$ , $k$ is a finite number. $a_1,a_2,...a_k$ are arbitary numbers.
Verbally, the series $a_n$ consists of an infinite number of periodic repetitions of the finite series $a_k.$
Finally my question is:
a) If there is an exist a algebraic closed form, for finite series $a_k$, in this case, does the $ a_n $ series always have a algebraic closed form?
b) If there is not an exist a algebraic closed form, for finite series $a_k$, in this case, does the $ a_n $ series always have a algebraic closed form?
I mean ,for example:
a)
$a_n=left{ 1,3,5,7,1,3,5,7,1,3,5,7,1,3,5,7,1,3,5,7cdotsright}$
b)
$a_n=left{1,8,2,6,5,9,1,8,2,6,5,9,1,8,2,6,5,9,1,8,2,6,5,9cdotsright}$
Thank you very much.
sequences-and-series soft-question closed-form
$endgroup$
|
show 8 more comments
$begingroup$
This question is a generalized form of the problem I asked before:
Algebraic Closed Form for $sum_{n=1}^{k}left( n- 3 lfloor frac{n-1}{3} rfloorright)$
Let, look at this periodic sequence:
$$a_n=left{a_1,a_2,a_3,a_4,a_5,cdots a_k ; a_1,a_2,a_3,a_4,a_5, cdots a_k; a_1,a_2,a_3,a_4,a_5,cdots a_k;cdots right}$$, where $left{a_1, a_2, a_3, cdots a_k right} in mathbb{Z^{+}}$ and $a_{k+1}=a_1, a_{k+2}=a_2, a_{k+3}=a_3, cdots a_{2k}=a_k, cdots$
For the sequence $a_k=left{ a_1,a_2,a_3,...,a_k right}$ , $k$ is a finite number. $a_1,a_2,...a_k$ are arbitary numbers.
Verbally, the series $a_n$ consists of an infinite number of periodic repetitions of the finite series $a_k.$
Finally my question is:
a) If there is an exist a algebraic closed form, for finite series $a_k$, in this case, does the $ a_n $ series always have a algebraic closed form?
b) If there is not an exist a algebraic closed form, for finite series $a_k$, in this case, does the $ a_n $ series always have a algebraic closed form?
I mean ,for example:
a)
$a_n=left{ 1,3,5,7,1,3,5,7,1,3,5,7,1,3,5,7,1,3,5,7cdotsright}$
b)
$a_n=left{1,8,2,6,5,9,1,8,2,6,5,9,1,8,2,6,5,9,1,8,2,6,5,9cdotsright}$
Thank you very much.
sequences-and-series soft-question closed-form
$endgroup$
$begingroup$
A finite sequence always does have a closed form, and so does a periodic sequence.
$endgroup$
– Robert Israel
Jan 8 at 12:39
$begingroup$
@RobertIsrael for any arbitary numbers for $a_1, a_2,a_3,...a_k$, There's an always closed form for $a_n$. Do I understand correct?
$endgroup$
– Elementary
Jan 8 at 12:46
$begingroup$
@Beginner, it depends on what you consider a "closed form". That term is not really well-defined. In a sense you have given a "closed form" in your definition of the sequence.
$endgroup$
– Mees de Vries
Jan 8 at 12:53
$begingroup$
@MeesdeVries I tried to give 2 example for a good understanding of the question..
$endgroup$
– Elementary
Jan 8 at 12:56
$begingroup$
This might be a closed form for your first sequence: $$ a_k = begin{cases} 1&text{if $k = 1 mod4$,}\ 3&text{if $k = 2 mod4$,}\ 5&text{if $k = 3 mod4$,}\ 7&text{else.} end{cases} $$ Does that satisfy what you're looking for in a "closed form"?
$endgroup$
– Mees de Vries
Jan 8 at 13:08
|
show 8 more comments
$begingroup$
This question is a generalized form of the problem I asked before:
Algebraic Closed Form for $sum_{n=1}^{k}left( n- 3 lfloor frac{n-1}{3} rfloorright)$
Let, look at this periodic sequence:
$$a_n=left{a_1,a_2,a_3,a_4,a_5,cdots a_k ; a_1,a_2,a_3,a_4,a_5, cdots a_k; a_1,a_2,a_3,a_4,a_5,cdots a_k;cdots right}$$, where $left{a_1, a_2, a_3, cdots a_k right} in mathbb{Z^{+}}$ and $a_{k+1}=a_1, a_{k+2}=a_2, a_{k+3}=a_3, cdots a_{2k}=a_k, cdots$
For the sequence $a_k=left{ a_1,a_2,a_3,...,a_k right}$ , $k$ is a finite number. $a_1,a_2,...a_k$ are arbitary numbers.
Verbally, the series $a_n$ consists of an infinite number of periodic repetitions of the finite series $a_k.$
Finally my question is:
a) If there is an exist a algebraic closed form, for finite series $a_k$, in this case, does the $ a_n $ series always have a algebraic closed form?
b) If there is not an exist a algebraic closed form, for finite series $a_k$, in this case, does the $ a_n $ series always have a algebraic closed form?
I mean ,for example:
a)
$a_n=left{ 1,3,5,7,1,3,5,7,1,3,5,7,1,3,5,7,1,3,5,7cdotsright}$
b)
$a_n=left{1,8,2,6,5,9,1,8,2,6,5,9,1,8,2,6,5,9,1,8,2,6,5,9cdotsright}$
Thank you very much.
sequences-and-series soft-question closed-form
$endgroup$
This question is a generalized form of the problem I asked before:
Algebraic Closed Form for $sum_{n=1}^{k}left( n- 3 lfloor frac{n-1}{3} rfloorright)$
Let, look at this periodic sequence:
$$a_n=left{a_1,a_2,a_3,a_4,a_5,cdots a_k ; a_1,a_2,a_3,a_4,a_5, cdots a_k; a_1,a_2,a_3,a_4,a_5,cdots a_k;cdots right}$$, where $left{a_1, a_2, a_3, cdots a_k right} in mathbb{Z^{+}}$ and $a_{k+1}=a_1, a_{k+2}=a_2, a_{k+3}=a_3, cdots a_{2k}=a_k, cdots$
For the sequence $a_k=left{ a_1,a_2,a_3,...,a_k right}$ , $k$ is a finite number. $a_1,a_2,...a_k$ are arbitary numbers.
Verbally, the series $a_n$ consists of an infinite number of periodic repetitions of the finite series $a_k.$
Finally my question is:
a) If there is an exist a algebraic closed form, for finite series $a_k$, in this case, does the $ a_n $ series always have a algebraic closed form?
b) If there is not an exist a algebraic closed form, for finite series $a_k$, in this case, does the $ a_n $ series always have a algebraic closed form?
I mean ,for example:
a)
$a_n=left{ 1,3,5,7,1,3,5,7,1,3,5,7,1,3,5,7,1,3,5,7cdotsright}$
b)
$a_n=left{1,8,2,6,5,9,1,8,2,6,5,9,1,8,2,6,5,9,1,8,2,6,5,9cdotsright}$
Thank you very much.
sequences-and-series soft-question closed-form
sequences-and-series soft-question closed-form
edited Jan 8 at 16:50
Elementary
asked Jan 8 at 12:36
ElementaryElementary
354111
354111
$begingroup$
A finite sequence always does have a closed form, and so does a periodic sequence.
$endgroup$
– Robert Israel
Jan 8 at 12:39
$begingroup$
@RobertIsrael for any arbitary numbers for $a_1, a_2,a_3,...a_k$, There's an always closed form for $a_n$. Do I understand correct?
$endgroup$
– Elementary
Jan 8 at 12:46
$begingroup$
@Beginner, it depends on what you consider a "closed form". That term is not really well-defined. In a sense you have given a "closed form" in your definition of the sequence.
$endgroup$
– Mees de Vries
Jan 8 at 12:53
$begingroup$
@MeesdeVries I tried to give 2 example for a good understanding of the question..
$endgroup$
– Elementary
Jan 8 at 12:56
$begingroup$
This might be a closed form for your first sequence: $$ a_k = begin{cases} 1&text{if $k = 1 mod4$,}\ 3&text{if $k = 2 mod4$,}\ 5&text{if $k = 3 mod4$,}\ 7&text{else.} end{cases} $$ Does that satisfy what you're looking for in a "closed form"?
$endgroup$
– Mees de Vries
Jan 8 at 13:08
|
show 8 more comments
$begingroup$
A finite sequence always does have a closed form, and so does a periodic sequence.
$endgroup$
– Robert Israel
Jan 8 at 12:39
$begingroup$
@RobertIsrael for any arbitary numbers for $a_1, a_2,a_3,...a_k$, There's an always closed form for $a_n$. Do I understand correct?
$endgroup$
– Elementary
Jan 8 at 12:46
$begingroup$
@Beginner, it depends on what you consider a "closed form". That term is not really well-defined. In a sense you have given a "closed form" in your definition of the sequence.
$endgroup$
– Mees de Vries
Jan 8 at 12:53
$begingroup$
@MeesdeVries I tried to give 2 example for a good understanding of the question..
$endgroup$
– Elementary
Jan 8 at 12:56
$begingroup$
This might be a closed form for your first sequence: $$ a_k = begin{cases} 1&text{if $k = 1 mod4$,}\ 3&text{if $k = 2 mod4$,}\ 5&text{if $k = 3 mod4$,}\ 7&text{else.} end{cases} $$ Does that satisfy what you're looking for in a "closed form"?
$endgroup$
– Mees de Vries
Jan 8 at 13:08
$begingroup$
A finite sequence always does have a closed form, and so does a periodic sequence.
$endgroup$
– Robert Israel
Jan 8 at 12:39
$begingroup$
A finite sequence always does have a closed form, and so does a periodic sequence.
$endgroup$
– Robert Israel
Jan 8 at 12:39
$begingroup$
@RobertIsrael for any arbitary numbers for $a_1, a_2,a_3,...a_k$, There's an always closed form for $a_n$. Do I understand correct?
$endgroup$
– Elementary
Jan 8 at 12:46
$begingroup$
@RobertIsrael for any arbitary numbers for $a_1, a_2,a_3,...a_k$, There's an always closed form for $a_n$. Do I understand correct?
$endgroup$
– Elementary
Jan 8 at 12:46
$begingroup$
@Beginner, it depends on what you consider a "closed form". That term is not really well-defined. In a sense you have given a "closed form" in your definition of the sequence.
$endgroup$
– Mees de Vries
Jan 8 at 12:53
$begingroup$
@Beginner, it depends on what you consider a "closed form". That term is not really well-defined. In a sense you have given a "closed form" in your definition of the sequence.
$endgroup$
– Mees de Vries
Jan 8 at 12:53
$begingroup$
@MeesdeVries I tried to give 2 example for a good understanding of the question..
$endgroup$
– Elementary
Jan 8 at 12:56
$begingroup$
@MeesdeVries I tried to give 2 example for a good understanding of the question..
$endgroup$
– Elementary
Jan 8 at 12:56
$begingroup$
This might be a closed form for your first sequence: $$ a_k = begin{cases} 1&text{if $k = 1 mod4$,}\ 3&text{if $k = 2 mod4$,}\ 5&text{if $k = 3 mod4$,}\ 7&text{else.} end{cases} $$ Does that satisfy what you're looking for in a "closed form"?
$endgroup$
– Mees de Vries
Jan 8 at 13:08
$begingroup$
This might be a closed form for your first sequence: $$ a_k = begin{cases} 1&text{if $k = 1 mod4$,}\ 3&text{if $k = 2 mod4$,}\ 5&text{if $k = 3 mod4$,}\ 7&text{else.} end{cases} $$ Does that satisfy what you're looking for in a "closed form"?
$endgroup$
– Mees de Vries
Jan 8 at 13:08
|
show 8 more comments
2 Answers
2
active
oldest
votes
$begingroup$
A periodic sequence with period $P$ can always be written as a trigonometric polynomial $$a_n = sum_{j=0}^{P-1} b_j cos(2 pi j n/P) + sum_{j=1}^{P-1} c_j sin(2 pi j n/P)$$
for some coefficients $b_j$ and $c_j$ (look up Finite Fourier Transform).
Thus your first example can be written as
$$a_n = 4-cos left( pi,j/2 right) -cos left( pi,j right) -cos
left( 3,pi,j/2 right) -sin left( pi,j/2 right) +sin
left( 3,pi,j/2 right)
$$
$endgroup$
add a comment |
$begingroup$
Let $f(x)$ be a polynomial interpolating $a_1,a_2,a_3,dots,a_k$.
Then the sequence is given by $a_n = f(1+((n-1) bmod k))$.
If you can't use mod directly, but can use floor, then note that
$$
a bmod b = a - b leftlfloor dfrac{a}{b} rightrfloor
$$
for $a,b in mathbb N$.
$endgroup$
add a comment |
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2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
votes
active
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active
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votes
$begingroup$
A periodic sequence with period $P$ can always be written as a trigonometric polynomial $$a_n = sum_{j=0}^{P-1} b_j cos(2 pi j n/P) + sum_{j=1}^{P-1} c_j sin(2 pi j n/P)$$
for some coefficients $b_j$ and $c_j$ (look up Finite Fourier Transform).
Thus your first example can be written as
$$a_n = 4-cos left( pi,j/2 right) -cos left( pi,j right) -cos
left( 3,pi,j/2 right) -sin left( pi,j/2 right) +sin
left( 3,pi,j/2 right)
$$
$endgroup$
add a comment |
$begingroup$
A periodic sequence with period $P$ can always be written as a trigonometric polynomial $$a_n = sum_{j=0}^{P-1} b_j cos(2 pi j n/P) + sum_{j=1}^{P-1} c_j sin(2 pi j n/P)$$
for some coefficients $b_j$ and $c_j$ (look up Finite Fourier Transform).
Thus your first example can be written as
$$a_n = 4-cos left( pi,j/2 right) -cos left( pi,j right) -cos
left( 3,pi,j/2 right) -sin left( pi,j/2 right) +sin
left( 3,pi,j/2 right)
$$
$endgroup$
add a comment |
$begingroup$
A periodic sequence with period $P$ can always be written as a trigonometric polynomial $$a_n = sum_{j=0}^{P-1} b_j cos(2 pi j n/P) + sum_{j=1}^{P-1} c_j sin(2 pi j n/P)$$
for some coefficients $b_j$ and $c_j$ (look up Finite Fourier Transform).
Thus your first example can be written as
$$a_n = 4-cos left( pi,j/2 right) -cos left( pi,j right) -cos
left( 3,pi,j/2 right) -sin left( pi,j/2 right) +sin
left( 3,pi,j/2 right)
$$
$endgroup$
A periodic sequence with period $P$ can always be written as a trigonometric polynomial $$a_n = sum_{j=0}^{P-1} b_j cos(2 pi j n/P) + sum_{j=1}^{P-1} c_j sin(2 pi j n/P)$$
for some coefficients $b_j$ and $c_j$ (look up Finite Fourier Transform).
Thus your first example can be written as
$$a_n = 4-cos left( pi,j/2 right) -cos left( pi,j right) -cos
left( 3,pi,j/2 right) -sin left( pi,j/2 right) +sin
left( 3,pi,j/2 right)
$$
edited Jan 8 at 16:59
answered Jan 8 at 16:39
Robert IsraelRobert Israel
324k23214468
324k23214468
add a comment |
add a comment |
$begingroup$
Let $f(x)$ be a polynomial interpolating $a_1,a_2,a_3,dots,a_k$.
Then the sequence is given by $a_n = f(1+((n-1) bmod k))$.
If you can't use mod directly, but can use floor, then note that
$$
a bmod b = a - b leftlfloor dfrac{a}{b} rightrfloor
$$
for $a,b in mathbb N$.
$endgroup$
add a comment |
$begingroup$
Let $f(x)$ be a polynomial interpolating $a_1,a_2,a_3,dots,a_k$.
Then the sequence is given by $a_n = f(1+((n-1) bmod k))$.
If you can't use mod directly, but can use floor, then note that
$$
a bmod b = a - b leftlfloor dfrac{a}{b} rightrfloor
$$
for $a,b in mathbb N$.
$endgroup$
add a comment |
$begingroup$
Let $f(x)$ be a polynomial interpolating $a_1,a_2,a_3,dots,a_k$.
Then the sequence is given by $a_n = f(1+((n-1) bmod k))$.
If you can't use mod directly, but can use floor, then note that
$$
a bmod b = a - b leftlfloor dfrac{a}{b} rightrfloor
$$
for $a,b in mathbb N$.
$endgroup$
Let $f(x)$ be a polynomial interpolating $a_1,a_2,a_3,dots,a_k$.
Then the sequence is given by $a_n = f(1+((n-1) bmod k))$.
If you can't use mod directly, but can use floor, then note that
$$
a bmod b = a - b leftlfloor dfrac{a}{b} rightrfloor
$$
for $a,b in mathbb N$.
edited Jan 8 at 15:13
answered Jan 8 at 13:08
lhflhf
165k10171396
165k10171396
add a comment |
add a comment |
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$begingroup$
A finite sequence always does have a closed form, and so does a periodic sequence.
$endgroup$
– Robert Israel
Jan 8 at 12:39
$begingroup$
@RobertIsrael for any arbitary numbers for $a_1, a_2,a_3,...a_k$, There's an always closed form for $a_n$. Do I understand correct?
$endgroup$
– Elementary
Jan 8 at 12:46
$begingroup$
@Beginner, it depends on what you consider a "closed form". That term is not really well-defined. In a sense you have given a "closed form" in your definition of the sequence.
$endgroup$
– Mees de Vries
Jan 8 at 12:53
$begingroup$
@MeesdeVries I tried to give 2 example for a good understanding of the question..
$endgroup$
– Elementary
Jan 8 at 12:56
$begingroup$
This might be a closed form for your first sequence: $$ a_k = begin{cases} 1&text{if $k = 1 mod4$,}\ 3&text{if $k = 2 mod4$,}\ 5&text{if $k = 3 mod4$,}\ 7&text{else.} end{cases} $$ Does that satisfy what you're looking for in a "closed form"?
$endgroup$
– Mees de Vries
Jan 8 at 13:08