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Number theory Difficulty 6.1 National olympiad Prove it Mongolia

a, b, c are integers and a<b<ca < b < c. An integer nn is said to be quadratical if a2+b2c2=na^2 + b^2 - c^2 = n. Find number of quadratical numbers no less than 1 and no greater than 2014.

Solution

Note that (3n)2+(4n)2=(5n2)(3n)^2 + (4n)^2 = (5n^2) from where we derive (3n)2+(4n1)2(5n1)2=2n(3n)^2 + (4n - 1)^2 - (5n - 1)^2 = 2n and (3n+2)2+(4n)2(5n1)2=2n+3(3n + 2)^2 + (4n)^2 - (5n - 1)^2 = 2n + 3.

a. Let's consider the case of quadratical number is odd. Set a=3n+2a = 3n + 2, b=4nb = 4n, c=5n+1c = 5n + 1 in the equality (3n+2)2+(4n)2(5n1)2=2n+3(3n + 2)^2 + (4n)^2 - (5n - 1)^2 = 2n + 3. If n3n \ge 3 then the condition a<b<ca < b < c holds. Therefore if m9m \ge 9 then any odd number m=2n+3m = 2n + 3 can be written in the form m=2n+3=a2+b2+c2m = 2n + 3 = a^2 + b^2 + c^2 as noted above. Now consider the cases m=1,3,5,7m = 1, 3, 5, 7.

1=42+72821 = 4^2 + 7^2 - 8^2, 3=42+62723 = 4^2 + 6^2 - 7^2, 5=42+52625 = 4^2 + 5^2 - 6^2, 7=102+1421727 = 10^2 + 14^2 - 17^2. Thus every odd number is quadratical.

b. Let's consider the case of quadratical number is even. Set a=3na = 3n, b=4n1b = 4n - 1, c=5n1c = 5n - 1 in the equality (3n)2+(4n1)2(5n1)2=2n(3n)^2 + (4n - 1)^2 - (5n - 1)^2 = 2n. If n2n \ge 2 then the condition a<b<ca < b < c holds. Note that 2=52+1121222 = 5^2 + 11^2 - 12^2 and from where we conclude that every even number is quadratical.

Hence all numbers 1 - 2014 are quadratical.

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