Integer part problem. [closed]
Clash Royale CLAN TAG#URR8PPP
up vote
0
down vote
favorite
Find all positive integers which $fracn+2017[sqrtn+1]$ and $fracn+2018[sqrtn]$ are natural numbers.
algebra-precalculus elementary-number-theory contest-math divisibility
closed as off-topic by Clarinetist, Michael Hoppe, 5xum, choco_addicted, Arthur Jul 30 at 12:40
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 improve the question by providing additional context, which ideally includes your thoughts on the problem and any attempts you have made to solve it. This information helps others identify where you have difficulties and helps them write answers appropriate to your experience level." – Clarinetist, Michael Hoppe, 5xum, choco_addicted, Arthur
add a comment |Â
up vote
0
down vote
favorite
Find all positive integers which $fracn+2017[sqrtn+1]$ and $fracn+2018[sqrtn]$ are natural numbers.
algebra-precalculus elementary-number-theory contest-math divisibility
closed as off-topic by Clarinetist, Michael Hoppe, 5xum, choco_addicted, Arthur Jul 30 at 12:40
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 improve the question by providing additional context, which ideally includes your thoughts on the problem and any attempts you have made to solve it. This information helps others identify where you have difficulties and helps them write answers appropriate to your experience level." – Clarinetist, Michael Hoppe, 5xum, choco_addicted, Arthur
That means that the numerator should be multiple of denominator...
– MysteryGuy
Jul 30 at 12:26
You use the tag contest-math. What contest is it, and is it over?
– Arthur
Jul 30 at 12:29
this problem was at romanian math olympiad - the local stage subject
– mathematiciangrade8
Jul 30 at 12:35
Do you use $[x]$ as the floor function or the nearest integer?
– kvantour
Jul 30 at 12:40
nearest integer
– mathematiciangrade8
Jul 30 at 12:40
add a comment |Â
up vote
0
down vote
favorite
up vote
0
down vote
favorite
Find all positive integers which $fracn+2017[sqrtn+1]$ and $fracn+2018[sqrtn]$ are natural numbers.
algebra-precalculus elementary-number-theory contest-math divisibility
Find all positive integers which $fracn+2017[sqrtn+1]$ and $fracn+2018[sqrtn]$ are natural numbers.
algebra-precalculus elementary-number-theory contest-math divisibility
edited Jul 30 at 12:37


greedoid
26.1k93473
26.1k93473
asked Jul 30 at 12:10
mathematiciangrade8
313
313
closed as off-topic by Clarinetist, Michael Hoppe, 5xum, choco_addicted, Arthur Jul 30 at 12:40
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 improve the question by providing additional context, which ideally includes your thoughts on the problem and any attempts you have made to solve it. This information helps others identify where you have difficulties and helps them write answers appropriate to your experience level." – Clarinetist, Michael Hoppe, 5xum, choco_addicted, Arthur
closed as off-topic by Clarinetist, Michael Hoppe, 5xum, choco_addicted, Arthur Jul 30 at 12:40
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 improve the question by providing additional context, which ideally includes your thoughts on the problem and any attempts you have made to solve it. This information helps others identify where you have difficulties and helps them write answers appropriate to your experience level." – Clarinetist, Michael Hoppe, 5xum, choco_addicted, Arthur
That means that the numerator should be multiple of denominator...
– MysteryGuy
Jul 30 at 12:26
You use the tag contest-math. What contest is it, and is it over?
– Arthur
Jul 30 at 12:29
this problem was at romanian math olympiad - the local stage subject
– mathematiciangrade8
Jul 30 at 12:35
Do you use $[x]$ as the floor function or the nearest integer?
– kvantour
Jul 30 at 12:40
nearest integer
– mathematiciangrade8
Jul 30 at 12:40
add a comment |Â
That means that the numerator should be multiple of denominator...
– MysteryGuy
Jul 30 at 12:26
You use the tag contest-math. What contest is it, and is it over?
– Arthur
Jul 30 at 12:29
this problem was at romanian math olympiad - the local stage subject
– mathematiciangrade8
Jul 30 at 12:35
Do you use $[x]$ as the floor function or the nearest integer?
– kvantour
Jul 30 at 12:40
nearest integer
– mathematiciangrade8
Jul 30 at 12:40
That means that the numerator should be multiple of denominator...
– MysteryGuy
Jul 30 at 12:26
That means that the numerator should be multiple of denominator...
– MysteryGuy
Jul 30 at 12:26
You use the tag contest-math. What contest is it, and is it over?
– Arthur
Jul 30 at 12:29
You use the tag contest-math. What contest is it, and is it over?
– Arthur
Jul 30 at 12:29
this problem was at romanian math olympiad - the local stage subject
– mathematiciangrade8
Jul 30 at 12:35
this problem was at romanian math olympiad - the local stage subject
– mathematiciangrade8
Jul 30 at 12:35
Do you use $[x]$ as the floor function or the nearest integer?
– kvantour
Jul 30 at 12:40
Do you use $[x]$ as the floor function or the nearest integer?
– kvantour
Jul 30 at 12:40
nearest integer
– mathematiciangrade8
Jul 30 at 12:40
nearest integer
– mathematiciangrade8
Jul 30 at 12:40
add a comment |Â
2 Answers
2
active
oldest
votes
up vote
2
down vote
Let $k$ be such an integer, that $$k^2leq n< (k+1)^2implies [sqrtn]=k$$
So $[sqrtn+1] in k,k+1$.
Case 1: $[sqrtn+1] =k$ then we have $kmid n+2017$ and $kmid n+2018$ so $$kmid (n+2018)-(n+2017)=1implies k=1$$
Thus $1leq nleq 3$...(easy to check which one is good)
Case 2: $[sqrtn+1] =k+1$ then we have $n = k^2+2k$ and so $$k+1mid k^2+2k+2017 = (k+1)^2+2016implies k+1mid 2016;;;;(*)$$ and $$kmid k^2+2k+2018 implies kmid 2018 implies kin1,2,1009,2018$$
Because of $(*)$ $1009$ and $2018$ clearly doesn't work.
So the solution are $n=1,2,3,8$.
i found that n=8 it respects
– mathematiciangrade8
Jul 30 at 12:31
Yes, I made an edit. If $k=2$ then $n=4+4=8$.
– greedoid
Jul 30 at 12:32
finally, u can write all solutions for n
– mathematiciangrade8
Jul 30 at 12:38
i found that solution but i forgot to say that the fractions over the problem can be natural numbers but not togheter at once!
– mathematiciangrade8
Jul 30 at 12:43
for example n=5 it respects the problem properties
– mathematiciangrade8
Jul 30 at 12:43
 |Â
show 6 more comments
up vote
1
down vote
Let $t:=lfloor sqrtnrfloor$. Clearly, $tleq lfloor sqrtn+1rfloor leq t+1$. If $lfloor sqrtn+1rfloor=t$, then $tmid n+2017$ and $tmid n+2018$ implies that $tmid 1$, whence $t=1$. That is, $nin1,2$.
If $lfloorsqrtn+1rfloor=t+1$, then $n=(t+1)^2-1$. Thus, $$t+1mid n+2017=(t+1)^2+2016text and tmid n+2018=(t+1)^2+2017,$$
That is, $t+1mid 2016$ and $tmid 2018$. Since the divisors of $2018$ are $1$, $2$, $1009$, and $2018$, we conclude that $tin1,2,1009,2018$. What next?
add a comment |Â
2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
votes
active
oldest
votes
active
oldest
votes
up vote
2
down vote
Let $k$ be such an integer, that $$k^2leq n< (k+1)^2implies [sqrtn]=k$$
So $[sqrtn+1] in k,k+1$.
Case 1: $[sqrtn+1] =k$ then we have $kmid n+2017$ and $kmid n+2018$ so $$kmid (n+2018)-(n+2017)=1implies k=1$$
Thus $1leq nleq 3$...(easy to check which one is good)
Case 2: $[sqrtn+1] =k+1$ then we have $n = k^2+2k$ and so $$k+1mid k^2+2k+2017 = (k+1)^2+2016implies k+1mid 2016;;;;(*)$$ and $$kmid k^2+2k+2018 implies kmid 2018 implies kin1,2,1009,2018$$
Because of $(*)$ $1009$ and $2018$ clearly doesn't work.
So the solution are $n=1,2,3,8$.
i found that n=8 it respects
– mathematiciangrade8
Jul 30 at 12:31
Yes, I made an edit. If $k=2$ then $n=4+4=8$.
– greedoid
Jul 30 at 12:32
finally, u can write all solutions for n
– mathematiciangrade8
Jul 30 at 12:38
i found that solution but i forgot to say that the fractions over the problem can be natural numbers but not togheter at once!
– mathematiciangrade8
Jul 30 at 12:43
for example n=5 it respects the problem properties
– mathematiciangrade8
Jul 30 at 12:43
 |Â
show 6 more comments
up vote
2
down vote
Let $k$ be such an integer, that $$k^2leq n< (k+1)^2implies [sqrtn]=k$$
So $[sqrtn+1] in k,k+1$.
Case 1: $[sqrtn+1] =k$ then we have $kmid n+2017$ and $kmid n+2018$ so $$kmid (n+2018)-(n+2017)=1implies k=1$$
Thus $1leq nleq 3$...(easy to check which one is good)
Case 2: $[sqrtn+1] =k+1$ then we have $n = k^2+2k$ and so $$k+1mid k^2+2k+2017 = (k+1)^2+2016implies k+1mid 2016;;;;(*)$$ and $$kmid k^2+2k+2018 implies kmid 2018 implies kin1,2,1009,2018$$
Because of $(*)$ $1009$ and $2018$ clearly doesn't work.
So the solution are $n=1,2,3,8$.
i found that n=8 it respects
– mathematiciangrade8
Jul 30 at 12:31
Yes, I made an edit. If $k=2$ then $n=4+4=8$.
– greedoid
Jul 30 at 12:32
finally, u can write all solutions for n
– mathematiciangrade8
Jul 30 at 12:38
i found that solution but i forgot to say that the fractions over the problem can be natural numbers but not togheter at once!
– mathematiciangrade8
Jul 30 at 12:43
for example n=5 it respects the problem properties
– mathematiciangrade8
Jul 30 at 12:43
 |Â
show 6 more comments
up vote
2
down vote
up vote
2
down vote
Let $k$ be such an integer, that $$k^2leq n< (k+1)^2implies [sqrtn]=k$$
So $[sqrtn+1] in k,k+1$.
Case 1: $[sqrtn+1] =k$ then we have $kmid n+2017$ and $kmid n+2018$ so $$kmid (n+2018)-(n+2017)=1implies k=1$$
Thus $1leq nleq 3$...(easy to check which one is good)
Case 2: $[sqrtn+1] =k+1$ then we have $n = k^2+2k$ and so $$k+1mid k^2+2k+2017 = (k+1)^2+2016implies k+1mid 2016;;;;(*)$$ and $$kmid k^2+2k+2018 implies kmid 2018 implies kin1,2,1009,2018$$
Because of $(*)$ $1009$ and $2018$ clearly doesn't work.
So the solution are $n=1,2,3,8$.
Let $k$ be such an integer, that $$k^2leq n< (k+1)^2implies [sqrtn]=k$$
So $[sqrtn+1] in k,k+1$.
Case 1: $[sqrtn+1] =k$ then we have $kmid n+2017$ and $kmid n+2018$ so $$kmid (n+2018)-(n+2017)=1implies k=1$$
Thus $1leq nleq 3$...(easy to check which one is good)
Case 2: $[sqrtn+1] =k+1$ then we have $n = k^2+2k$ and so $$k+1mid k^2+2k+2017 = (k+1)^2+2016implies k+1mid 2016;;;;(*)$$ and $$kmid k^2+2k+2018 implies kmid 2018 implies kin1,2,1009,2018$$
Because of $(*)$ $1009$ and $2018$ clearly doesn't work.
So the solution are $n=1,2,3,8$.
edited Jul 30 at 12:41
answered Jul 30 at 12:17


greedoid
26.1k93473
26.1k93473
i found that n=8 it respects
– mathematiciangrade8
Jul 30 at 12:31
Yes, I made an edit. If $k=2$ then $n=4+4=8$.
– greedoid
Jul 30 at 12:32
finally, u can write all solutions for n
– mathematiciangrade8
Jul 30 at 12:38
i found that solution but i forgot to say that the fractions over the problem can be natural numbers but not togheter at once!
– mathematiciangrade8
Jul 30 at 12:43
for example n=5 it respects the problem properties
– mathematiciangrade8
Jul 30 at 12:43
 |Â
show 6 more comments
i found that n=8 it respects
– mathematiciangrade8
Jul 30 at 12:31
Yes, I made an edit. If $k=2$ then $n=4+4=8$.
– greedoid
Jul 30 at 12:32
finally, u can write all solutions for n
– mathematiciangrade8
Jul 30 at 12:38
i found that solution but i forgot to say that the fractions over the problem can be natural numbers but not togheter at once!
– mathematiciangrade8
Jul 30 at 12:43
for example n=5 it respects the problem properties
– mathematiciangrade8
Jul 30 at 12:43
i found that n=8 it respects
– mathematiciangrade8
Jul 30 at 12:31
i found that n=8 it respects
– mathematiciangrade8
Jul 30 at 12:31
Yes, I made an edit. If $k=2$ then $n=4+4=8$.
– greedoid
Jul 30 at 12:32
Yes, I made an edit. If $k=2$ then $n=4+4=8$.
– greedoid
Jul 30 at 12:32
finally, u can write all solutions for n
– mathematiciangrade8
Jul 30 at 12:38
finally, u can write all solutions for n
– mathematiciangrade8
Jul 30 at 12:38
i found that solution but i forgot to say that the fractions over the problem can be natural numbers but not togheter at once!
– mathematiciangrade8
Jul 30 at 12:43
i found that solution but i forgot to say that the fractions over the problem can be natural numbers but not togheter at once!
– mathematiciangrade8
Jul 30 at 12:43
for example n=5 it respects the problem properties
– mathematiciangrade8
Jul 30 at 12:43
for example n=5 it respects the problem properties
– mathematiciangrade8
Jul 30 at 12:43
 |Â
show 6 more comments
up vote
1
down vote
Let $t:=lfloor sqrtnrfloor$. Clearly, $tleq lfloor sqrtn+1rfloor leq t+1$. If $lfloor sqrtn+1rfloor=t$, then $tmid n+2017$ and $tmid n+2018$ implies that $tmid 1$, whence $t=1$. That is, $nin1,2$.
If $lfloorsqrtn+1rfloor=t+1$, then $n=(t+1)^2-1$. Thus, $$t+1mid n+2017=(t+1)^2+2016text and tmid n+2018=(t+1)^2+2017,$$
That is, $t+1mid 2016$ and $tmid 2018$. Since the divisors of $2018$ are $1$, $2$, $1009$, and $2018$, we conclude that $tin1,2,1009,2018$. What next?
add a comment |Â
up vote
1
down vote
Let $t:=lfloor sqrtnrfloor$. Clearly, $tleq lfloor sqrtn+1rfloor leq t+1$. If $lfloor sqrtn+1rfloor=t$, then $tmid n+2017$ and $tmid n+2018$ implies that $tmid 1$, whence $t=1$. That is, $nin1,2$.
If $lfloorsqrtn+1rfloor=t+1$, then $n=(t+1)^2-1$. Thus, $$t+1mid n+2017=(t+1)^2+2016text and tmid n+2018=(t+1)^2+2017,$$
That is, $t+1mid 2016$ and $tmid 2018$. Since the divisors of $2018$ are $1$, $2$, $1009$, and $2018$, we conclude that $tin1,2,1009,2018$. What next?
add a comment |Â
up vote
1
down vote
up vote
1
down vote
Let $t:=lfloor sqrtnrfloor$. Clearly, $tleq lfloor sqrtn+1rfloor leq t+1$. If $lfloor sqrtn+1rfloor=t$, then $tmid n+2017$ and $tmid n+2018$ implies that $tmid 1$, whence $t=1$. That is, $nin1,2$.
If $lfloorsqrtn+1rfloor=t+1$, then $n=(t+1)^2-1$. Thus, $$t+1mid n+2017=(t+1)^2+2016text and tmid n+2018=(t+1)^2+2017,$$
That is, $t+1mid 2016$ and $tmid 2018$. Since the divisors of $2018$ are $1$, $2$, $1009$, and $2018$, we conclude that $tin1,2,1009,2018$. What next?
Let $t:=lfloor sqrtnrfloor$. Clearly, $tleq lfloor sqrtn+1rfloor leq t+1$. If $lfloor sqrtn+1rfloor=t$, then $tmid n+2017$ and $tmid n+2018$ implies that $tmid 1$, whence $t=1$. That is, $nin1,2$.
If $lfloorsqrtn+1rfloor=t+1$, then $n=(t+1)^2-1$. Thus, $$t+1mid n+2017=(t+1)^2+2016text and tmid n+2018=(t+1)^2+2017,$$
That is, $t+1mid 2016$ and $tmid 2018$. Since the divisors of $2018$ are $1$, $2$, $1009$, and $2018$, we conclude that $tin1,2,1009,2018$. What next?
edited Jul 30 at 12:35
answered Jul 30 at 12:29


Batominovski
22.9k22777
22.9k22777
add a comment |Â
add a comment |Â
That means that the numerator should be multiple of denominator...
– MysteryGuy
Jul 30 at 12:26
You use the tag contest-math. What contest is it, and is it over?
– Arthur
Jul 30 at 12:29
this problem was at romanian math olympiad - the local stage subject
– mathematiciangrade8
Jul 30 at 12:35
Do you use $[x]$ as the floor function or the nearest integer?
– kvantour
Jul 30 at 12:40
nearest integer
– mathematiciangrade8
Jul 30 at 12:40