Sunday, September 06, 2020

Two Math Problems of interest (at least to me)

 I will give two math problems that are of interest to me.

These are not new problems, however you will have more fun if you work on them yourself and leave comments on what you find. So if you want to work on it without hints, don't read the comments.

I will post about the answers (not sure I will post THE answers) on Thursday.

1) Let x(1)>0. Let x(n+1) = (  1 + (1/x(n))  )^n. 

For how many values of x(1) does this sequence go to infinity?

2) Find all (x,y) \in N \times N such that x^2+3y and y^2+3x are both squares. 


  1. #1 is missing a parenthesis somewhere.

  2. My idea about #2 ... probably not enough "elementary":

    1. GREAT, THANKS- your solution works, so READERS- if you want to
      do it yourself do not read his the solution pointed to.

  3. For #2: Assume wlog that x>=y. Then x^2+3y < x^2+4x+4 = (x+2)^2,
    and also x^2+3y > x^2. Since the only perfect square strictly
    between x^2 and (x+2)^2 is (x+1)^2, we get x^2+3y=(x+1)^2.

    This yields 3y=2x+1, so that y=2a+1 is odd, and so that x=3a+1.
    Furthermore y^2+3x = 4a^2+13a+4 =:Q.

    Now Q >= (2a+2)^2 = 4a^2+8a+4 and Q < (2a+4)^2 = 4a^2+16a+16.
    This implies that either Q=(2a+2)^2 with a=0 and x=y=1,
    or that Q=(2a+3)^2 with a=5, x=16 and y=11. Done.