Given a Fibonacci number , find the next Fibonacci numberhow to find nth term in a fibonacci series or sum of...

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Given a Fibonacci number , find the next Fibonacci number


how to find nth term in a fibonacci series or sum of a series of fibonacci numbersdetermine the number of terms in a fibonacci sequence that are divisible by $3$How do I choose first terms of a Fibonacci sequence?Is fibonacci sequence a member of more broad family of sequences?First Fibonacci Number with Given RemainderNumber of ways to write $n$ as sum of odd or even number of Fibonacci numbersModification of the Fibonacci Sequences






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6














$begingroup$


The Fibonacci sequence is $0, 1, 1, 2, 3, 5, 8, 13, 21, 34,ldots$, where each term after the first two is the sum of the two previous terms.



Can we find the next Fibonacci number if we are given any Fibonacci number?



For example, if $n = 8$ then it's answer should be $13$ because $13$ is the next Fibonacci number after $8$.










share|cite|improve this question












$endgroup$






















    6














    $begingroup$


    The Fibonacci sequence is $0, 1, 1, 2, 3, 5, 8, 13, 21, 34,ldots$, where each term after the first two is the sum of the two previous terms.



    Can we find the next Fibonacci number if we are given any Fibonacci number?



    For example, if $n = 8$ then it's answer should be $13$ because $13$ is the next Fibonacci number after $8$.










    share|cite|improve this question












    $endgroup$


















      6












      6








      6


      4



      $begingroup$


      The Fibonacci sequence is $0, 1, 1, 2, 3, 5, 8, 13, 21, 34,ldots$, where each term after the first two is the sum of the two previous terms.



      Can we find the next Fibonacci number if we are given any Fibonacci number?



      For example, if $n = 8$ then it's answer should be $13$ because $13$ is the next Fibonacci number after $8$.










      share|cite|improve this question












      $endgroup$




      The Fibonacci sequence is $0, 1, 1, 2, 3, 5, 8, 13, 21, 34,ldots$, where each term after the first two is the sum of the two previous terms.



      Can we find the next Fibonacci number if we are given any Fibonacci number?



      For example, if $n = 8$ then it's answer should be $13$ because $13$ is the next Fibonacci number after $8$.







      sequences-and-series combinatorics discrete-mathematics fibonacci-numbers






      share|cite|improve this question
















      share|cite|improve this question













      share|cite|improve this question




      share|cite|improve this question








      edited 1 hour ago









      Matthew Daly

      7,0301 gold badge10 silver badges29 bronze badges




      7,0301 gold badge10 silver badges29 bronze badges










      asked yesterday









      sr123sr123

      966 bronze badges




      966 bronze badges

























          3 Answers
          3






          active

          oldest

          votes


















          20
















          $begingroup$

          The ratio of any two consecutive entries in the Fibonacci sequence rapidly approaches $varphi=frac{1+sqrt5}2$. So if you multiply your number by $frac{1+sqrt5}2$ and round to the nearest integer, you will get the next term unless you're at the very beginning of the sequence.






          share|cite|improve this answer












          $endgroup$















          • $begingroup$
            float ans = ((1+sqrt(5))/2.0)*n; return ceil(ans); This is not giving correct result.
            $endgroup$
            – sr123
            yesterday








          • 17




            $begingroup$
            @sr123 He did say "round to the nearest integer". That's not what ceil does, so of course you don't get the correct result. From checking the first few values, actually rounding will get you correct results unless you're starting from 0 or from 1.
            $endgroup$
            – HTNW
            19 hours ago










          • $begingroup$
            yes float ans = ((1+sqrt(5))/2.0)*n; return round(ans); is working !!
            $endgroup$
            – sr123
            13 hours ago






          • 3




            $begingroup$
            Trying to do this in floating point is a bad idea; Fibonacci numbers quickly grow beyond the ability of floating point to accurately represent, so your computation will suffer from rounding error.
            $endgroup$
            – user2357112
            11 hours ago






          • 6




            $begingroup$
            This is a math site, not a computer site, and we work with abstract real number arithmetic here, not floating point. However, anyone actually trying to compute a Fibonacci number by multiplying another Fibonacci number by $(1+sqrt{5})/2$ is probably going to use a computer, and representation issues will be important.
            $endgroup$
            – user2357112
            5 hours ago



















          15
















          $begingroup$

          $ninmathbb{N}$ is a Fibonacci number iff $5n^2-4$ or $5n^2+4$ is a square. In the former case $n=F_{2k+1}$ while in the latter case $n=F_{2k}$. Assuming $ngeq 2$, in the former case $F_{2k+2}=lfloor varphi n rfloor$ and in the latter case $F_{2k+1}=lceil varphi nrceil $, with $varphi=frac{1+sqrt{5}}{2}$.



          Example: if $n=8$ we have that $5cdot 8^2+4=18^2$, hence $n$ is a Fibonacci number with even index and the next Fibonacci number is $lceil 8varphi rceil =13$.






          share|cite|improve this answer










          $endgroup$











          • 4




            $begingroup$
            If you've gone to the trouble to find the square root $m$ of $5n^2 - 4$ or $5n^2 + 4$, you can avoid floating-point arithmetic by computing the next Fibonacci number as $(n + m)/2$.
            $endgroup$
            – Mark Dickinson
            8 hours ago










          • $begingroup$
            @MarkDickinson How does that work?
            $endgroup$
            – Varad Mahashabde
            2 hours ago






          • 1




            $begingroup$
            Good question, but it looks like @TonyK has saved me from answering ...
            $endgroup$
            – Mark Dickinson
            2 hours ago



















          8
















          $begingroup$

          Given a Fibonacci number $n$, let $m$ be the next Fibonacci number. The Fibonacci sequence has the property that for any three consecutive elements $r,s,t$, we have $rt-s^2=pm 1$ (proof is by induction, which you might like to try $-$ the choice of signs alternates). And we know that the previous Fibonacci number is $m-n$. So we have
          $$m(m-n)=n^2pm 1$$
          This is a quadratic equation in $m$, with solutions
          $m=frac12(npmsqrt{5n^2pm 4})$. We know that $m>n$, so $m$ must equal $frac12(n+sqrt{5n^2pm 4})$. And we can choose between $+4$ and $-4$ because only one of $sqrt{5n^2+4}$ and $sqrt{5n^2-4}$ can be an integer (with the single exception of $n=1$).



          So the answer is whichever one of $frac12(n+sqrt{5n^2+4})$ and $frac12(n+sqrt{5n^2-4})$ is an integer.



          Note that the single exception $n=1$ occurs twice in the Fibonacci sequence, so there are indeed two possible answers.






          share|cite|improve this answer












          $endgroup$

















            Your Answer








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            3 Answers
            3






            active

            oldest

            votes








            3 Answers
            3






            active

            oldest

            votes









            active

            oldest

            votes






            active

            oldest

            votes









            20
















            $begingroup$

            The ratio of any two consecutive entries in the Fibonacci sequence rapidly approaches $varphi=frac{1+sqrt5}2$. So if you multiply your number by $frac{1+sqrt5}2$ and round to the nearest integer, you will get the next term unless you're at the very beginning of the sequence.






            share|cite|improve this answer












            $endgroup$















            • $begingroup$
              float ans = ((1+sqrt(5))/2.0)*n; return ceil(ans); This is not giving correct result.
              $endgroup$
              – sr123
              yesterday








            • 17




              $begingroup$
              @sr123 He did say "round to the nearest integer". That's not what ceil does, so of course you don't get the correct result. From checking the first few values, actually rounding will get you correct results unless you're starting from 0 or from 1.
              $endgroup$
              – HTNW
              19 hours ago










            • $begingroup$
              yes float ans = ((1+sqrt(5))/2.0)*n; return round(ans); is working !!
              $endgroup$
              – sr123
              13 hours ago






            • 3




              $begingroup$
              Trying to do this in floating point is a bad idea; Fibonacci numbers quickly grow beyond the ability of floating point to accurately represent, so your computation will suffer from rounding error.
              $endgroup$
              – user2357112
              11 hours ago






            • 6




              $begingroup$
              This is a math site, not a computer site, and we work with abstract real number arithmetic here, not floating point. However, anyone actually trying to compute a Fibonacci number by multiplying another Fibonacci number by $(1+sqrt{5})/2$ is probably going to use a computer, and representation issues will be important.
              $endgroup$
              – user2357112
              5 hours ago
















            20
















            $begingroup$

            The ratio of any two consecutive entries in the Fibonacci sequence rapidly approaches $varphi=frac{1+sqrt5}2$. So if you multiply your number by $frac{1+sqrt5}2$ and round to the nearest integer, you will get the next term unless you're at the very beginning of the sequence.






            share|cite|improve this answer












            $endgroup$















            • $begingroup$
              float ans = ((1+sqrt(5))/2.0)*n; return ceil(ans); This is not giving correct result.
              $endgroup$
              – sr123
              yesterday








            • 17




              $begingroup$
              @sr123 He did say "round to the nearest integer". That's not what ceil does, so of course you don't get the correct result. From checking the first few values, actually rounding will get you correct results unless you're starting from 0 or from 1.
              $endgroup$
              – HTNW
              19 hours ago










            • $begingroup$
              yes float ans = ((1+sqrt(5))/2.0)*n; return round(ans); is working !!
              $endgroup$
              – sr123
              13 hours ago






            • 3




              $begingroup$
              Trying to do this in floating point is a bad idea; Fibonacci numbers quickly grow beyond the ability of floating point to accurately represent, so your computation will suffer from rounding error.
              $endgroup$
              – user2357112
              11 hours ago






            • 6




              $begingroup$
              This is a math site, not a computer site, and we work with abstract real number arithmetic here, not floating point. However, anyone actually trying to compute a Fibonacci number by multiplying another Fibonacci number by $(1+sqrt{5})/2$ is probably going to use a computer, and representation issues will be important.
              $endgroup$
              – user2357112
              5 hours ago














            20














            20










            20







            $begingroup$

            The ratio of any two consecutive entries in the Fibonacci sequence rapidly approaches $varphi=frac{1+sqrt5}2$. So if you multiply your number by $frac{1+sqrt5}2$ and round to the nearest integer, you will get the next term unless you're at the very beginning of the sequence.






            share|cite|improve this answer












            $endgroup$



            The ratio of any two consecutive entries in the Fibonacci sequence rapidly approaches $varphi=frac{1+sqrt5}2$. So if you multiply your number by $frac{1+sqrt5}2$ and round to the nearest integer, you will get the next term unless you're at the very beginning of the sequence.







            share|cite|improve this answer















            share|cite|improve this answer




            share|cite|improve this answer








            edited 1 hour ago

























            answered yesterday









            Matthew DalyMatthew Daly

            7,0301 gold badge10 silver badges29 bronze badges




            7,0301 gold badge10 silver badges29 bronze badges















            • $begingroup$
              float ans = ((1+sqrt(5))/2.0)*n; return ceil(ans); This is not giving correct result.
              $endgroup$
              – sr123
              yesterday








            • 17




              $begingroup$
              @sr123 He did say "round to the nearest integer". That's not what ceil does, so of course you don't get the correct result. From checking the first few values, actually rounding will get you correct results unless you're starting from 0 or from 1.
              $endgroup$
              – HTNW
              19 hours ago










            • $begingroup$
              yes float ans = ((1+sqrt(5))/2.0)*n; return round(ans); is working !!
              $endgroup$
              – sr123
              13 hours ago






            • 3




              $begingroup$
              Trying to do this in floating point is a bad idea; Fibonacci numbers quickly grow beyond the ability of floating point to accurately represent, so your computation will suffer from rounding error.
              $endgroup$
              – user2357112
              11 hours ago






            • 6




              $begingroup$
              This is a math site, not a computer site, and we work with abstract real number arithmetic here, not floating point. However, anyone actually trying to compute a Fibonacci number by multiplying another Fibonacci number by $(1+sqrt{5})/2$ is probably going to use a computer, and representation issues will be important.
              $endgroup$
              – user2357112
              5 hours ago


















            • $begingroup$
              float ans = ((1+sqrt(5))/2.0)*n; return ceil(ans); This is not giving correct result.
              $endgroup$
              – sr123
              yesterday








            • 17




              $begingroup$
              @sr123 He did say "round to the nearest integer". That's not what ceil does, so of course you don't get the correct result. From checking the first few values, actually rounding will get you correct results unless you're starting from 0 or from 1.
              $endgroup$
              – HTNW
              19 hours ago










            • $begingroup$
              yes float ans = ((1+sqrt(5))/2.0)*n; return round(ans); is working !!
              $endgroup$
              – sr123
              13 hours ago






            • 3




              $begingroup$
              Trying to do this in floating point is a bad idea; Fibonacci numbers quickly grow beyond the ability of floating point to accurately represent, so your computation will suffer from rounding error.
              $endgroup$
              – user2357112
              11 hours ago






            • 6




              $begingroup$
              This is a math site, not a computer site, and we work with abstract real number arithmetic here, not floating point. However, anyone actually trying to compute a Fibonacci number by multiplying another Fibonacci number by $(1+sqrt{5})/2$ is probably going to use a computer, and representation issues will be important.
              $endgroup$
              – user2357112
              5 hours ago
















            $begingroup$
            float ans = ((1+sqrt(5))/2.0)*n; return ceil(ans); This is not giving correct result.
            $endgroup$
            – sr123
            yesterday






            $begingroup$
            float ans = ((1+sqrt(5))/2.0)*n; return ceil(ans); This is not giving correct result.
            $endgroup$
            – sr123
            yesterday






            17




            17




            $begingroup$
            @sr123 He did say "round to the nearest integer". That's not what ceil does, so of course you don't get the correct result. From checking the first few values, actually rounding will get you correct results unless you're starting from 0 or from 1.
            $endgroup$
            – HTNW
            19 hours ago




            $begingroup$
            @sr123 He did say "round to the nearest integer". That's not what ceil does, so of course you don't get the correct result. From checking the first few values, actually rounding will get you correct results unless you're starting from 0 or from 1.
            $endgroup$
            – HTNW
            19 hours ago












            $begingroup$
            yes float ans = ((1+sqrt(5))/2.0)*n; return round(ans); is working !!
            $endgroup$
            – sr123
            13 hours ago




            $begingroup$
            yes float ans = ((1+sqrt(5))/2.0)*n; return round(ans); is working !!
            $endgroup$
            – sr123
            13 hours ago




            3




            3




            $begingroup$
            Trying to do this in floating point is a bad idea; Fibonacci numbers quickly grow beyond the ability of floating point to accurately represent, so your computation will suffer from rounding error.
            $endgroup$
            – user2357112
            11 hours ago




            $begingroup$
            Trying to do this in floating point is a bad idea; Fibonacci numbers quickly grow beyond the ability of floating point to accurately represent, so your computation will suffer from rounding error.
            $endgroup$
            – user2357112
            11 hours ago




            6




            6




            $begingroup$
            This is a math site, not a computer site, and we work with abstract real number arithmetic here, not floating point. However, anyone actually trying to compute a Fibonacci number by multiplying another Fibonacci number by $(1+sqrt{5})/2$ is probably going to use a computer, and representation issues will be important.
            $endgroup$
            – user2357112
            5 hours ago




            $begingroup$
            This is a math site, not a computer site, and we work with abstract real number arithmetic here, not floating point. However, anyone actually trying to compute a Fibonacci number by multiplying another Fibonacci number by $(1+sqrt{5})/2$ is probably going to use a computer, and representation issues will be important.
            $endgroup$
            – user2357112
            5 hours ago













            15
















            $begingroup$

            $ninmathbb{N}$ is a Fibonacci number iff $5n^2-4$ or $5n^2+4$ is a square. In the former case $n=F_{2k+1}$ while in the latter case $n=F_{2k}$. Assuming $ngeq 2$, in the former case $F_{2k+2}=lfloor varphi n rfloor$ and in the latter case $F_{2k+1}=lceil varphi nrceil $, with $varphi=frac{1+sqrt{5}}{2}$.



            Example: if $n=8$ we have that $5cdot 8^2+4=18^2$, hence $n$ is a Fibonacci number with even index and the next Fibonacci number is $lceil 8varphi rceil =13$.






            share|cite|improve this answer










            $endgroup$











            • 4




              $begingroup$
              If you've gone to the trouble to find the square root $m$ of $5n^2 - 4$ or $5n^2 + 4$, you can avoid floating-point arithmetic by computing the next Fibonacci number as $(n + m)/2$.
              $endgroup$
              – Mark Dickinson
              8 hours ago










            • $begingroup$
              @MarkDickinson How does that work?
              $endgroup$
              – Varad Mahashabde
              2 hours ago






            • 1




              $begingroup$
              Good question, but it looks like @TonyK has saved me from answering ...
              $endgroup$
              – Mark Dickinson
              2 hours ago
















            15
















            $begingroup$

            $ninmathbb{N}$ is a Fibonacci number iff $5n^2-4$ or $5n^2+4$ is a square. In the former case $n=F_{2k+1}$ while in the latter case $n=F_{2k}$. Assuming $ngeq 2$, in the former case $F_{2k+2}=lfloor varphi n rfloor$ and in the latter case $F_{2k+1}=lceil varphi nrceil $, with $varphi=frac{1+sqrt{5}}{2}$.



            Example: if $n=8$ we have that $5cdot 8^2+4=18^2$, hence $n$ is a Fibonacci number with even index and the next Fibonacci number is $lceil 8varphi rceil =13$.






            share|cite|improve this answer










            $endgroup$











            • 4




              $begingroup$
              If you've gone to the trouble to find the square root $m$ of $5n^2 - 4$ or $5n^2 + 4$, you can avoid floating-point arithmetic by computing the next Fibonacci number as $(n + m)/2$.
              $endgroup$
              – Mark Dickinson
              8 hours ago










            • $begingroup$
              @MarkDickinson How does that work?
              $endgroup$
              – Varad Mahashabde
              2 hours ago






            • 1




              $begingroup$
              Good question, but it looks like @TonyK has saved me from answering ...
              $endgroup$
              – Mark Dickinson
              2 hours ago














            15














            15










            15







            $begingroup$

            $ninmathbb{N}$ is a Fibonacci number iff $5n^2-4$ or $5n^2+4$ is a square. In the former case $n=F_{2k+1}$ while in the latter case $n=F_{2k}$. Assuming $ngeq 2$, in the former case $F_{2k+2}=lfloor varphi n rfloor$ and in the latter case $F_{2k+1}=lceil varphi nrceil $, with $varphi=frac{1+sqrt{5}}{2}$.



            Example: if $n=8$ we have that $5cdot 8^2+4=18^2$, hence $n$ is a Fibonacci number with even index and the next Fibonacci number is $lceil 8varphi rceil =13$.






            share|cite|improve this answer










            $endgroup$



            $ninmathbb{N}$ is a Fibonacci number iff $5n^2-4$ or $5n^2+4$ is a square. In the former case $n=F_{2k+1}$ while in the latter case $n=F_{2k}$. Assuming $ngeq 2$, in the former case $F_{2k+2}=lfloor varphi n rfloor$ and in the latter case $F_{2k+1}=lceil varphi nrceil $, with $varphi=frac{1+sqrt{5}}{2}$.



            Example: if $n=8$ we have that $5cdot 8^2+4=18^2$, hence $n$ is a Fibonacci number with even index and the next Fibonacci number is $lceil 8varphi rceil =13$.







            share|cite|improve this answer













            share|cite|improve this answer




            share|cite|improve this answer










            answered yesterday









            Jack D'AurizioJack D'Aurizio

            302k33 gold badges297 silver badges695 bronze badges




            302k33 gold badges297 silver badges695 bronze badges











            • 4




              $begingroup$
              If you've gone to the trouble to find the square root $m$ of $5n^2 - 4$ or $5n^2 + 4$, you can avoid floating-point arithmetic by computing the next Fibonacci number as $(n + m)/2$.
              $endgroup$
              – Mark Dickinson
              8 hours ago










            • $begingroup$
              @MarkDickinson How does that work?
              $endgroup$
              – Varad Mahashabde
              2 hours ago






            • 1




              $begingroup$
              Good question, but it looks like @TonyK has saved me from answering ...
              $endgroup$
              – Mark Dickinson
              2 hours ago














            • 4




              $begingroup$
              If you've gone to the trouble to find the square root $m$ of $5n^2 - 4$ or $5n^2 + 4$, you can avoid floating-point arithmetic by computing the next Fibonacci number as $(n + m)/2$.
              $endgroup$
              – Mark Dickinson
              8 hours ago










            • $begingroup$
              @MarkDickinson How does that work?
              $endgroup$
              – Varad Mahashabde
              2 hours ago






            • 1




              $begingroup$
              Good question, but it looks like @TonyK has saved me from answering ...
              $endgroup$
              – Mark Dickinson
              2 hours ago








            4




            4




            $begingroup$
            If you've gone to the trouble to find the square root $m$ of $5n^2 - 4$ or $5n^2 + 4$, you can avoid floating-point arithmetic by computing the next Fibonacci number as $(n + m)/2$.
            $endgroup$
            – Mark Dickinson
            8 hours ago




            $begingroup$
            If you've gone to the trouble to find the square root $m$ of $5n^2 - 4$ or $5n^2 + 4$, you can avoid floating-point arithmetic by computing the next Fibonacci number as $(n + m)/2$.
            $endgroup$
            – Mark Dickinson
            8 hours ago












            $begingroup$
            @MarkDickinson How does that work?
            $endgroup$
            – Varad Mahashabde
            2 hours ago




            $begingroup$
            @MarkDickinson How does that work?
            $endgroup$
            – Varad Mahashabde
            2 hours ago




            1




            1




            $begingroup$
            Good question, but it looks like @TonyK has saved me from answering ...
            $endgroup$
            – Mark Dickinson
            2 hours ago




            $begingroup$
            Good question, but it looks like @TonyK has saved me from answering ...
            $endgroup$
            – Mark Dickinson
            2 hours ago











            8
















            $begingroup$

            Given a Fibonacci number $n$, let $m$ be the next Fibonacci number. The Fibonacci sequence has the property that for any three consecutive elements $r,s,t$, we have $rt-s^2=pm 1$ (proof is by induction, which you might like to try $-$ the choice of signs alternates). And we know that the previous Fibonacci number is $m-n$. So we have
            $$m(m-n)=n^2pm 1$$
            This is a quadratic equation in $m$, with solutions
            $m=frac12(npmsqrt{5n^2pm 4})$. We know that $m>n$, so $m$ must equal $frac12(n+sqrt{5n^2pm 4})$. And we can choose between $+4$ and $-4$ because only one of $sqrt{5n^2+4}$ and $sqrt{5n^2-4}$ can be an integer (with the single exception of $n=1$).



            So the answer is whichever one of $frac12(n+sqrt{5n^2+4})$ and $frac12(n+sqrt{5n^2-4})$ is an integer.



            Note that the single exception $n=1$ occurs twice in the Fibonacci sequence, so there are indeed two possible answers.






            share|cite|improve this answer












            $endgroup$




















              8
















              $begingroup$

              Given a Fibonacci number $n$, let $m$ be the next Fibonacci number. The Fibonacci sequence has the property that for any three consecutive elements $r,s,t$, we have $rt-s^2=pm 1$ (proof is by induction, which you might like to try $-$ the choice of signs alternates). And we know that the previous Fibonacci number is $m-n$. So we have
              $$m(m-n)=n^2pm 1$$
              This is a quadratic equation in $m$, with solutions
              $m=frac12(npmsqrt{5n^2pm 4})$. We know that $m>n$, so $m$ must equal $frac12(n+sqrt{5n^2pm 4})$. And we can choose between $+4$ and $-4$ because only one of $sqrt{5n^2+4}$ and $sqrt{5n^2-4}$ can be an integer (with the single exception of $n=1$).



              So the answer is whichever one of $frac12(n+sqrt{5n^2+4})$ and $frac12(n+sqrt{5n^2-4})$ is an integer.



              Note that the single exception $n=1$ occurs twice in the Fibonacci sequence, so there are indeed two possible answers.






              share|cite|improve this answer












              $endgroup$


















                8














                8










                8







                $begingroup$

                Given a Fibonacci number $n$, let $m$ be the next Fibonacci number. The Fibonacci sequence has the property that for any three consecutive elements $r,s,t$, we have $rt-s^2=pm 1$ (proof is by induction, which you might like to try $-$ the choice of signs alternates). And we know that the previous Fibonacci number is $m-n$. So we have
                $$m(m-n)=n^2pm 1$$
                This is a quadratic equation in $m$, with solutions
                $m=frac12(npmsqrt{5n^2pm 4})$. We know that $m>n$, so $m$ must equal $frac12(n+sqrt{5n^2pm 4})$. And we can choose between $+4$ and $-4$ because only one of $sqrt{5n^2+4}$ and $sqrt{5n^2-4}$ can be an integer (with the single exception of $n=1$).



                So the answer is whichever one of $frac12(n+sqrt{5n^2+4})$ and $frac12(n+sqrt{5n^2-4})$ is an integer.



                Note that the single exception $n=1$ occurs twice in the Fibonacci sequence, so there are indeed two possible answers.






                share|cite|improve this answer












                $endgroup$



                Given a Fibonacci number $n$, let $m$ be the next Fibonacci number. The Fibonacci sequence has the property that for any three consecutive elements $r,s,t$, we have $rt-s^2=pm 1$ (proof is by induction, which you might like to try $-$ the choice of signs alternates). And we know that the previous Fibonacci number is $m-n$. So we have
                $$m(m-n)=n^2pm 1$$
                This is a quadratic equation in $m$, with solutions
                $m=frac12(npmsqrt{5n^2pm 4})$. We know that $m>n$, so $m$ must equal $frac12(n+sqrt{5n^2pm 4})$. And we can choose between $+4$ and $-4$ because only one of $sqrt{5n^2+4}$ and $sqrt{5n^2-4}$ can be an integer (with the single exception of $n=1$).



                So the answer is whichever one of $frac12(n+sqrt{5n^2+4})$ and $frac12(n+sqrt{5n^2-4})$ is an integer.



                Note that the single exception $n=1$ occurs twice in the Fibonacci sequence, so there are indeed two possible answers.







                share|cite|improve this answer















                share|cite|improve this answer




                share|cite|improve this answer








                edited 1 hour ago

























                answered 3 hours ago









                TonyKTonyK

                47.4k3 gold badges62 silver badges140 bronze badges




                47.4k3 gold badges62 silver badges140 bronze badges


































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