Are there infinitely-many numbers that are both square and triangular?

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I just started to read "Friendly Introduction to Number Theory" but I am getting stuck in the first exercises.




1.1. The first two numbers that are both squares and triangles are 1 and 36. Find the next one and, if possible, the one after that. Can
you figure out an efficient way to find triangular–square numbers? Do
you think that there are infinitely many?




https://www.math.brown.edu/~jhs/frintch1ch6.pdf



I found how to find out the number which is both square and triangle. (don't know if this is effective way)



https://github.com/y-zono/friendly-introduction-number-theory/blob/master/01/1-1/main.go



However how can I answer "Do you think that there are infinitely many?"? I think I need to find the formula but no idea yet. Can you please help me?










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    I just started to read "Friendly Introduction to Number Theory" but I am getting stuck in the first exercises.




    1.1. The first two numbers that are both squares and triangles are 1 and 36. Find the next one and, if possible, the one after that. Can
    you figure out an efficient way to find triangular–square numbers? Do
    you think that there are infinitely many?




    https://www.math.brown.edu/~jhs/frintch1ch6.pdf



    I found how to find out the number which is both square and triangle. (don't know if this is effective way)



    https://github.com/y-zono/friendly-introduction-number-theory/blob/master/01/1-1/main.go



    However how can I answer "Do you think that there are infinitely many?"? I think I need to find the formula but no idea yet. Can you please help me?










    share|cite|improve this question









    New contributor




    zono is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
    Check out our Code of Conduct.





















      up vote
      1
      down vote

      favorite









      up vote
      1
      down vote

      favorite











      I just started to read "Friendly Introduction to Number Theory" but I am getting stuck in the first exercises.




      1.1. The first two numbers that are both squares and triangles are 1 and 36. Find the next one and, if possible, the one after that. Can
      you figure out an efficient way to find triangular–square numbers? Do
      you think that there are infinitely many?




      https://www.math.brown.edu/~jhs/frintch1ch6.pdf



      I found how to find out the number which is both square and triangle. (don't know if this is effective way)



      https://github.com/y-zono/friendly-introduction-number-theory/blob/master/01/1-1/main.go



      However how can I answer "Do you think that there are infinitely many?"? I think I need to find the formula but no idea yet. Can you please help me?










      share|cite|improve this question









      New contributor




      zono is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
      Check out our Code of Conduct.











      I just started to read "Friendly Introduction to Number Theory" but I am getting stuck in the first exercises.




      1.1. The first two numbers that are both squares and triangles are 1 and 36. Find the next one and, if possible, the one after that. Can
      you figure out an efficient way to find triangular–square numbers? Do
      you think that there are infinitely many?




      https://www.math.brown.edu/~jhs/frintch1ch6.pdf



      I found how to find out the number which is both square and triangle. (don't know if this is effective way)



      https://github.com/y-zono/friendly-introduction-number-theory/blob/master/01/1-1/main.go



      However how can I answer "Do you think that there are infinitely many?"? I think I need to find the formula but no idea yet. Can you please help me?







      number-theory






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      edited 2 hours ago









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      asked 2 hours ago









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          4 Answers
          4






          active

          oldest

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          up vote
          2
          down vote













          Have a look at https://en.wikipedia.org/wiki/Square_triangular_number
          So, the answere is yes!






          share|cite|improve this answer








          New contributor




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          • Thank you for great reference. I did not know the term "Square triangular number".
            – zono
            1 hour ago


















          up vote
          1
          down vote













          The formula for triangle numbers is on page 9. Create an equation with the left being that formula using x, and the right the formula for squares, using y. X and y are not equal, but you can rearrange terms. Ask, when is the left divisible by something on the right? Are we sure that such divisibility exists for higher numbers?



          Better, assume we have such an x and y. Can we always add something to each to get the next number?






          share|cite|improve this answer




















          • Thank you! Now I'm trying to understand what you are saying... not easy for me. (my English skills is not good too)
            – zono
            1 hour ago










          • Good luck to you. The other answers are very detailed, but you have to do only enough to answer the question, are there infinitely many? I read the chapter, and it wants you to think about things that we can show go on forever, and other things that are conjectures (twin primes is one... we think it goes on forever but we do not know.)
            – Russ
            1 hour ago











          • Also, thank you for trying before asking, and welcome to the board! :-)
            – Russ
            1 hour ago

















          up vote
          1
          down vote













          I think there are infinite such numbers, but they're rarer and rarer the farther the number from zero.
          My computer gave these results:
          1,
          6, 35, 204, 1189, 6930., 40391, 235416. 1372105,
          (The numbers which on squaring give triangular numbers until 10^7)






          share|cite|improve this answer








          New contributor




          Harshith Vasireddy is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
          Check out our Code of Conduct.

















          • Thank you for answering. I agree that there are infinite such numbers. It took for a while on my PC to show the numbers.
            – zono
            2 hours ago

















          up vote
          1
          down vote













          Of course the solution of equation:



          $$Y^2=fracX(Xpm1)2$$



          Defined solutions of Pell's equation: $$p^2-2s^2=pm1$$



          But it is necessary to write the formula describing their solutions through solving Pell's equation:



          $$X=p^2+4ps+4s^2$$



          $$Y=p^2+3ps+2s^2$$



          And more.



          $$X=2s^2$$



          $$Y=ps$$



          $p,s$ - These numbers can be any character.
          If you need to have a solution of the equation: $$Y^2=fracX(Xpma)2$$



          It is necessary to substitute into the formulas uravneniyaPellya solutions: $$p^2-2s^2=pma$$






          share|cite|improve this answer




















          • Thank you so much! Give me for a while to understand it. So many information for me. I think it will take few days..
            – zono
            1 hour ago










          Your Answer




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          4 Answers
          4






          active

          oldest

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          4 Answers
          4






          active

          oldest

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          active

          oldest

          votes






          active

          oldest

          votes








          up vote
          2
          down vote













          Have a look at https://en.wikipedia.org/wiki/Square_triangular_number
          So, the answere is yes!






          share|cite|improve this answer








          New contributor




          ALG is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
          Check out our Code of Conduct.

















          • Thank you for great reference. I did not know the term "Square triangular number".
            – zono
            1 hour ago















          up vote
          2
          down vote













          Have a look at https://en.wikipedia.org/wiki/Square_triangular_number
          So, the answere is yes!






          share|cite|improve this answer








          New contributor




          ALG is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
          Check out our Code of Conduct.

















          • Thank you for great reference. I did not know the term "Square triangular number".
            – zono
            1 hour ago













          up vote
          2
          down vote










          up vote
          2
          down vote









          Have a look at https://en.wikipedia.org/wiki/Square_triangular_number
          So, the answere is yes!






          share|cite|improve this answer








          New contributor




          ALG is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
          Check out our Code of Conduct.









          Have a look at https://en.wikipedia.org/wiki/Square_triangular_number
          So, the answere is yes!







          share|cite|improve this answer








          New contributor




          ALG is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
          Check out our Code of Conduct.









          share|cite|improve this answer



          share|cite|improve this answer






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          answered 2 hours ago









          ALG

          561




          561




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          • Thank you for great reference. I did not know the term "Square triangular number".
            – zono
            1 hour ago

















          • Thank you for great reference. I did not know the term "Square triangular number".
            – zono
            1 hour ago
















          Thank you for great reference. I did not know the term "Square triangular number".
          – zono
          1 hour ago





          Thank you for great reference. I did not know the term "Square triangular number".
          – zono
          1 hour ago











          up vote
          1
          down vote













          The formula for triangle numbers is on page 9. Create an equation with the left being that formula using x, and the right the formula for squares, using y. X and y are not equal, but you can rearrange terms. Ask, when is the left divisible by something on the right? Are we sure that such divisibility exists for higher numbers?



          Better, assume we have such an x and y. Can we always add something to each to get the next number?






          share|cite|improve this answer




















          • Thank you! Now I'm trying to understand what you are saying... not easy for me. (my English skills is not good too)
            – zono
            1 hour ago










          • Good luck to you. The other answers are very detailed, but you have to do only enough to answer the question, are there infinitely many? I read the chapter, and it wants you to think about things that we can show go on forever, and other things that are conjectures (twin primes is one... we think it goes on forever but we do not know.)
            – Russ
            1 hour ago











          • Also, thank you for trying before asking, and welcome to the board! :-)
            – Russ
            1 hour ago














          up vote
          1
          down vote













          The formula for triangle numbers is on page 9. Create an equation with the left being that formula using x, and the right the formula for squares, using y. X and y are not equal, but you can rearrange terms. Ask, when is the left divisible by something on the right? Are we sure that such divisibility exists for higher numbers?



          Better, assume we have such an x and y. Can we always add something to each to get the next number?






          share|cite|improve this answer




















          • Thank you! Now I'm trying to understand what you are saying... not easy for me. (my English skills is not good too)
            – zono
            1 hour ago










          • Good luck to you. The other answers are very detailed, but you have to do only enough to answer the question, are there infinitely many? I read the chapter, and it wants you to think about things that we can show go on forever, and other things that are conjectures (twin primes is one... we think it goes on forever but we do not know.)
            – Russ
            1 hour ago











          • Also, thank you for trying before asking, and welcome to the board! :-)
            – Russ
            1 hour ago












          up vote
          1
          down vote










          up vote
          1
          down vote









          The formula for triangle numbers is on page 9. Create an equation with the left being that formula using x, and the right the formula for squares, using y. X and y are not equal, but you can rearrange terms. Ask, when is the left divisible by something on the right? Are we sure that such divisibility exists for higher numbers?



          Better, assume we have such an x and y. Can we always add something to each to get the next number?






          share|cite|improve this answer












          The formula for triangle numbers is on page 9. Create an equation with the left being that formula using x, and the right the formula for squares, using y. X and y are not equal, but you can rearrange terms. Ask, when is the left divisible by something on the right? Are we sure that such divisibility exists for higher numbers?



          Better, assume we have such an x and y. Can we always add something to each to get the next number?







          share|cite|improve this answer












          share|cite|improve this answer



          share|cite|improve this answer










          answered 2 hours ago









          Russ

          478




          478











          • Thank you! Now I'm trying to understand what you are saying... not easy for me. (my English skills is not good too)
            – zono
            1 hour ago










          • Good luck to you. The other answers are very detailed, but you have to do only enough to answer the question, are there infinitely many? I read the chapter, and it wants you to think about things that we can show go on forever, and other things that are conjectures (twin primes is one... we think it goes on forever but we do not know.)
            – Russ
            1 hour ago











          • Also, thank you for trying before asking, and welcome to the board! :-)
            – Russ
            1 hour ago
















          • Thank you! Now I'm trying to understand what you are saying... not easy for me. (my English skills is not good too)
            – zono
            1 hour ago










          • Good luck to you. The other answers are very detailed, but you have to do only enough to answer the question, are there infinitely many? I read the chapter, and it wants you to think about things that we can show go on forever, and other things that are conjectures (twin primes is one... we think it goes on forever but we do not know.)
            – Russ
            1 hour ago











          • Also, thank you for trying before asking, and welcome to the board! :-)
            – Russ
            1 hour ago















          Thank you! Now I'm trying to understand what you are saying... not easy for me. (my English skills is not good too)
          – zono
          1 hour ago




          Thank you! Now I'm trying to understand what you are saying... not easy for me. (my English skills is not good too)
          – zono
          1 hour ago












          Good luck to you. The other answers are very detailed, but you have to do only enough to answer the question, are there infinitely many? I read the chapter, and it wants you to think about things that we can show go on forever, and other things that are conjectures (twin primes is one... we think it goes on forever but we do not know.)
          – Russ
          1 hour ago





          Good luck to you. The other answers are very detailed, but you have to do only enough to answer the question, are there infinitely many? I read the chapter, and it wants you to think about things that we can show go on forever, and other things that are conjectures (twin primes is one... we think it goes on forever but we do not know.)
          – Russ
          1 hour ago













          Also, thank you for trying before asking, and welcome to the board! :-)
          – Russ
          1 hour ago




          Also, thank you for trying before asking, and welcome to the board! :-)
          – Russ
          1 hour ago










          up vote
          1
          down vote













          I think there are infinite such numbers, but they're rarer and rarer the farther the number from zero.
          My computer gave these results:
          1,
          6, 35, 204, 1189, 6930., 40391, 235416. 1372105,
          (The numbers which on squaring give triangular numbers until 10^7)






          share|cite|improve this answer








          New contributor




          Harshith Vasireddy is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
          Check out our Code of Conduct.

















          • Thank you for answering. I agree that there are infinite such numbers. It took for a while on my PC to show the numbers.
            – zono
            2 hours ago














          up vote
          1
          down vote













          I think there are infinite such numbers, but they're rarer and rarer the farther the number from zero.
          My computer gave these results:
          1,
          6, 35, 204, 1189, 6930., 40391, 235416. 1372105,
          (The numbers which on squaring give triangular numbers until 10^7)






          share|cite|improve this answer








          New contributor




          Harshith Vasireddy is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
          Check out our Code of Conduct.

















          • Thank you for answering. I agree that there are infinite such numbers. It took for a while on my PC to show the numbers.
            – zono
            2 hours ago












          up vote
          1
          down vote










          up vote
          1
          down vote









          I think there are infinite such numbers, but they're rarer and rarer the farther the number from zero.
          My computer gave these results:
          1,
          6, 35, 204, 1189, 6930., 40391, 235416. 1372105,
          (The numbers which on squaring give triangular numbers until 10^7)






          share|cite|improve this answer








          New contributor




          Harshith Vasireddy is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
          Check out our Code of Conduct.









          I think there are infinite such numbers, but they're rarer and rarer the farther the number from zero.
          My computer gave these results:
          1,
          6, 35, 204, 1189, 6930., 40391, 235416. 1372105,
          (The numbers which on squaring give triangular numbers until 10^7)







          share|cite|improve this answer








          New contributor




          Harshith Vasireddy is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
          Check out our Code of Conduct.









          share|cite|improve this answer



          share|cite|improve this answer






          New contributor




          Harshith Vasireddy is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
          Check out our Code of Conduct.









          answered 2 hours ago









          Harshith Vasireddy

          111




          111




          New contributor




          Harshith Vasireddy is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
          Check out our Code of Conduct.





          New contributor





          Harshith Vasireddy is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
          Check out our Code of Conduct.






          Harshith Vasireddy is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
          Check out our Code of Conduct.











          • Thank you for answering. I agree that there are infinite such numbers. It took for a while on my PC to show the numbers.
            – zono
            2 hours ago
















          • Thank you for answering. I agree that there are infinite such numbers. It took for a while on my PC to show the numbers.
            – zono
            2 hours ago















          Thank you for answering. I agree that there are infinite such numbers. It took for a while on my PC to show the numbers.
          – zono
          2 hours ago




          Thank you for answering. I agree that there are infinite such numbers. It took for a while on my PC to show the numbers.
          – zono
          2 hours ago










          up vote
          1
          down vote













          Of course the solution of equation:



          $$Y^2=fracX(Xpm1)2$$



          Defined solutions of Pell's equation: $$p^2-2s^2=pm1$$



          But it is necessary to write the formula describing their solutions through solving Pell's equation:



          $$X=p^2+4ps+4s^2$$



          $$Y=p^2+3ps+2s^2$$



          And more.



          $$X=2s^2$$



          $$Y=ps$$



          $p,s$ - These numbers can be any character.
          If you need to have a solution of the equation: $$Y^2=fracX(Xpma)2$$



          It is necessary to substitute into the formulas uravneniyaPellya solutions: $$p^2-2s^2=pma$$






          share|cite|improve this answer




















          • Thank you so much! Give me for a while to understand it. So many information for me. I think it will take few days..
            – zono
            1 hour ago














          up vote
          1
          down vote













          Of course the solution of equation:



          $$Y^2=fracX(Xpm1)2$$



          Defined solutions of Pell's equation: $$p^2-2s^2=pm1$$



          But it is necessary to write the formula describing their solutions through solving Pell's equation:



          $$X=p^2+4ps+4s^2$$



          $$Y=p^2+3ps+2s^2$$



          And more.



          $$X=2s^2$$



          $$Y=ps$$



          $p,s$ - These numbers can be any character.
          If you need to have a solution of the equation: $$Y^2=fracX(Xpma)2$$



          It is necessary to substitute into the formulas uravneniyaPellya solutions: $$p^2-2s^2=pma$$






          share|cite|improve this answer




















          • Thank you so much! Give me for a while to understand it. So many information for me. I think it will take few days..
            – zono
            1 hour ago












          up vote
          1
          down vote










          up vote
          1
          down vote









          Of course the solution of equation:



          $$Y^2=fracX(Xpm1)2$$



          Defined solutions of Pell's equation: $$p^2-2s^2=pm1$$



          But it is necessary to write the formula describing their solutions through solving Pell's equation:



          $$X=p^2+4ps+4s^2$$



          $$Y=p^2+3ps+2s^2$$



          And more.



          $$X=2s^2$$



          $$Y=ps$$



          $p,s$ - These numbers can be any character.
          If you need to have a solution of the equation: $$Y^2=fracX(Xpma)2$$



          It is necessary to substitute into the formulas uravneniyaPellya solutions: $$p^2-2s^2=pma$$






          share|cite|improve this answer












          Of course the solution of equation:



          $$Y^2=fracX(Xpm1)2$$



          Defined solutions of Pell's equation: $$p^2-2s^2=pm1$$



          But it is necessary to write the formula describing their solutions through solving Pell's equation:



          $$X=p^2+4ps+4s^2$$



          $$Y=p^2+3ps+2s^2$$



          And more.



          $$X=2s^2$$



          $$Y=ps$$



          $p,s$ - These numbers can be any character.
          If you need to have a solution of the equation: $$Y^2=fracX(Xpma)2$$



          It is necessary to substitute into the formulas uravneniyaPellya solutions: $$p^2-2s^2=pma$$







          share|cite|improve this answer












          share|cite|improve this answer



          share|cite|improve this answer










          answered 2 hours ago









          individ

          3,2141715




          3,2141715











          • Thank you so much! Give me for a while to understand it. So many information for me. I think it will take few days..
            – zono
            1 hour ago
















          • Thank you so much! Give me for a while to understand it. So many information for me. I think it will take few days..
            – zono
            1 hour ago















          Thank you so much! Give me for a while to understand it. So many information for me. I think it will take few days..
          – zono
          1 hour ago




          Thank you so much! Give me for a while to understand it. So many information for me. I think it will take few days..
          – zono
          1 hour ago










          zono is a new contributor. Be nice, and check out our Code of Conduct.









           

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