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Integer Rep (Posted on 2005-11-24) Difficulty: 2 of 5
Show that for any integer n there exists integers i, j, and k such that

n = i2 + j2 - 5k2

L.E. Dickson

  Submitted by Bractals    
Rating: 3.5000 (2 votes)
Solution: (Hide)
Here are some of the infinite number of solutions for even or odd n:

    (m - a1)2 + (2m - b1)2 - 5(m - c1)2 = 2m = n

    (m - a2)2 + (2m - b2)2 - 5(m - c2)2 = 2m + 1 = n

where

     a1 = 2(5k2 + 5k + 1)

     b1 = 20k2 + 10(2 - t)k + (6 - 5t)

     c1 = 10k2 + 2(5 - 2t)k + (3 - 2t)


     a2 = 10k2 - 1

     b2 = 10k(2k + t)

     c2 = 2k(5k + 2t)

with t = +1 or -1  and k any integer.

Here are some examples with small values for t and k:
    

    (m - 2)2 + (2m - 1)2 - 5(m - 1)2 = 2m = n              t = -1 and k = -1

    (m - 9)2 + (2m - 30)2 - 5(m - 14)2 = 2m + 1 = n                "

    (m - 2)2 + (2m - 11)2 - 5(m - 5)2 = 2m = n             t = 1 and k = -1

    (m - 9)2 + (2m - 10)2 - 5(m - 6)2 = 2m + 1 = n                 "

    (m - 2)2 + (2m - 11)2 - 5(m - 5)2 = 2m = n             t = -1 and k = 0

    (m + 1)2 + (2m - 0)2 - 5(m - 0)2 = 2m + 1 = n                  "

    (m - 2)2 + (2m - 1)2 - 5(m - 1)2 = 2m = n              t = 1 and k = 0

    (m + 1)2 + (2m - 0)2 - 5(m - 0)2 = 2m + 1 = n                  "

    (m - 22)2 + (2m - 61)2 - 5(m - 29)2 = 2m = n           t = -1 and k = 1

    (m - 9)2 + (2m - 10)2 - 5(m - 6)2 = 2m + 1 = n                 "

    (m - 22)2 + (2m - 31)2 - 5(m - 17)2 = 2m = n           t = 1 and k = 1

    (m - 9)2 + (2m - 30)2 - 5(m - 14)2 = 2m + 1 = n                "

Comments: ( You must be logged in to post comments.)
  Subject Author Date
re: Solution (thanks)Mindy Rodriguez2005-11-25 13:08:32
SolutionGamer2005-11-24 22:20:05
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