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Diophantine equation 3^a+1=3^b+5^c



Planned maintenance scheduled April 23, 2019 at 23:30 UTC (7:30pm US/Eastern)
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1












$begingroup$


This is not a research problem, but challenging enough that I've decided to post it in here:



Determine all triples $(a,b,c)$ of non-negative integers, satisfying
$$
1+3^a = 3^b+5^c.
$$










share|cite|improve this question









$endgroup$

















    1












    $begingroup$


    This is not a research problem, but challenging enough that I've decided to post it in here:



    Determine all triples $(a,b,c)$ of non-negative integers, satisfying
    $$
    1+3^a = 3^b+5^c.
    $$










    share|cite|improve this question









    $endgroup$















      1












      1








      1





      $begingroup$


      This is not a research problem, but challenging enough that I've decided to post it in here:



      Determine all triples $(a,b,c)$ of non-negative integers, satisfying
      $$
      1+3^a = 3^b+5^c.
      $$










      share|cite|improve this question









      $endgroup$




      This is not a research problem, but challenging enough that I've decided to post it in here:



      Determine all triples $(a,b,c)$ of non-negative integers, satisfying
      $$
      1+3^a = 3^b+5^c.
      $$







      nt.number-theory diophantine-equations elementary-proofs






      share|cite|improve this question













      share|cite|improve this question











      share|cite|improve this question




      share|cite|improve this question










      asked 1 hour ago









      kawakawa

      1707




      1707






















          1 Answer
          1






          active

          oldest

          votes


















          5












          $begingroup$

          I can't resist this: The young Chris Skinner showed that if $a$, $b$, $c$, $d$ are fixed positive integers, and $p$ and $q$ are positive coprime integers then the equation
          $$
          ap^x + bq^y = c+ dp^z q^w
          $$

          has a bounded number of solutions in $(x,y,z,w)$ and that a bound on these could be computed (and the equation solved in practice). This solves (in principle) the more general equation $1+3^a 5^d = 3^b+ 5^c$. Anyway, there is a large literature around such exponential diophantine equations, and Skinner's paper will give some references.



          Following a reference from Skinner's paper, Theorem 4.01 of Brenner and Foster gives an explicit treatment of the equation
          $$
          3^a + 7^b=3^c+5^d,
          $$

          which completely resolves the problem in this question (take $b=0$). Their proof is elementary, and the only non-trivial solution to the equation in the question is $3^3+1 = 3 + 5^2$.






          share|cite|improve this answer











          $endgroup$













          • $begingroup$
            Lucia, many thanks for the paper.
            $endgroup$
            – kawa
            36 mins ago












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          1 Answer
          1






          active

          oldest

          votes









          active

          oldest

          votes






          active

          oldest

          votes









          5












          $begingroup$

          I can't resist this: The young Chris Skinner showed that if $a$, $b$, $c$, $d$ are fixed positive integers, and $p$ and $q$ are positive coprime integers then the equation
          $$
          ap^x + bq^y = c+ dp^z q^w
          $$

          has a bounded number of solutions in $(x,y,z,w)$ and that a bound on these could be computed (and the equation solved in practice). This solves (in principle) the more general equation $1+3^a 5^d = 3^b+ 5^c$. Anyway, there is a large literature around such exponential diophantine equations, and Skinner's paper will give some references.



          Following a reference from Skinner's paper, Theorem 4.01 of Brenner and Foster gives an explicit treatment of the equation
          $$
          3^a + 7^b=3^c+5^d,
          $$

          which completely resolves the problem in this question (take $b=0$). Their proof is elementary, and the only non-trivial solution to the equation in the question is $3^3+1 = 3 + 5^2$.






          share|cite|improve this answer











          $endgroup$













          • $begingroup$
            Lucia, many thanks for the paper.
            $endgroup$
            – kawa
            36 mins ago
















          5












          $begingroup$

          I can't resist this: The young Chris Skinner showed that if $a$, $b$, $c$, $d$ are fixed positive integers, and $p$ and $q$ are positive coprime integers then the equation
          $$
          ap^x + bq^y = c+ dp^z q^w
          $$

          has a bounded number of solutions in $(x,y,z,w)$ and that a bound on these could be computed (and the equation solved in practice). This solves (in principle) the more general equation $1+3^a 5^d = 3^b+ 5^c$. Anyway, there is a large literature around such exponential diophantine equations, and Skinner's paper will give some references.



          Following a reference from Skinner's paper, Theorem 4.01 of Brenner and Foster gives an explicit treatment of the equation
          $$
          3^a + 7^b=3^c+5^d,
          $$

          which completely resolves the problem in this question (take $b=0$). Their proof is elementary, and the only non-trivial solution to the equation in the question is $3^3+1 = 3 + 5^2$.






          share|cite|improve this answer











          $endgroup$













          • $begingroup$
            Lucia, many thanks for the paper.
            $endgroup$
            – kawa
            36 mins ago














          5












          5








          5





          $begingroup$

          I can't resist this: The young Chris Skinner showed that if $a$, $b$, $c$, $d$ are fixed positive integers, and $p$ and $q$ are positive coprime integers then the equation
          $$
          ap^x + bq^y = c+ dp^z q^w
          $$

          has a bounded number of solutions in $(x,y,z,w)$ and that a bound on these could be computed (and the equation solved in practice). This solves (in principle) the more general equation $1+3^a 5^d = 3^b+ 5^c$. Anyway, there is a large literature around such exponential diophantine equations, and Skinner's paper will give some references.



          Following a reference from Skinner's paper, Theorem 4.01 of Brenner and Foster gives an explicit treatment of the equation
          $$
          3^a + 7^b=3^c+5^d,
          $$

          which completely resolves the problem in this question (take $b=0$). Their proof is elementary, and the only non-trivial solution to the equation in the question is $3^3+1 = 3 + 5^2$.






          share|cite|improve this answer











          $endgroup$



          I can't resist this: The young Chris Skinner showed that if $a$, $b$, $c$, $d$ are fixed positive integers, and $p$ and $q$ are positive coprime integers then the equation
          $$
          ap^x + bq^y = c+ dp^z q^w
          $$

          has a bounded number of solutions in $(x,y,z,w)$ and that a bound on these could be computed (and the equation solved in practice). This solves (in principle) the more general equation $1+3^a 5^d = 3^b+ 5^c$. Anyway, there is a large literature around such exponential diophantine equations, and Skinner's paper will give some references.



          Following a reference from Skinner's paper, Theorem 4.01 of Brenner and Foster gives an explicit treatment of the equation
          $$
          3^a + 7^b=3^c+5^d,
          $$

          which completely resolves the problem in this question (take $b=0$). Their proof is elementary, and the only non-trivial solution to the equation in the question is $3^3+1 = 3 + 5^2$.







          share|cite|improve this answer














          share|cite|improve this answer



          share|cite|improve this answer








          edited 11 mins ago

























          answered 40 mins ago









          LuciaLucia

          35k5151177




          35k5151177












          • $begingroup$
            Lucia, many thanks for the paper.
            $endgroup$
            – kawa
            36 mins ago


















          • $begingroup$
            Lucia, many thanks for the paper.
            $endgroup$
            – kawa
            36 mins ago
















          $begingroup$
          Lucia, many thanks for the paper.
          $endgroup$
          – kawa
          36 mins ago




          $begingroup$
          Lucia, many thanks for the paper.
          $endgroup$
          – kawa
          36 mins ago


















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