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A square pattern? (Posted on 2020-09-27) Difficulty: 2 of 5
Is the expression (14+12+1)(24+22+1)...(n4+n2+1) a square for any natural number?

No Solution Yet Submitted by Danish Ahmed Khan    
Rating: 3.0000 (2 votes)

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Some Thoughts Possible solution | Comment 1 of 3
Generalisation for all numbers n, (n+1), (n+2) etc.

Consider small numbers of terms:

(n^4+n^2+1) = (n^2-n+1)(n^2+n+1)

(n^4+n^2+1)((n+1)^4+(n+1)^2+1) = (n^2-n+1)(n^2+n+1)^2(n^2+3n+3)

(n^4+n^2+1)((n+1)^4+(n+1)^2+1)((n+2)^4+(n+2)^2+1) = 
(n^2-n+1)(n^2+n+1)^2(n^2+3n+3)^2(n^2+5n+7)

(n^4+n^2+1)((n+1)^4+(n+1)^2+1)((n+2)^4+(n+2)^2+1)((n+3)^4+(n+3)^2+1) = 
(n^2-n+1)(n^2+n+1)^2(n^2+3n+3)^2(n^2+5n+7)^2(n^2+7n+13)

(n^4+n^2+1)((n+1)^4+(n+1)^2+1)((n+2)^4+(n+2)^2+1)((n+3)^4+(n+3)^2+1)((n+4)^4+(n+4)^2+1) =
(n^2-n+1)(n^2+n+1)^2(n^2+3n+3)^2(n^2+5n+7)^2(n^2+7n+13)^2(n^2+9n+21) 

The intermediate expressions, except for the first and last, form a series of squares, so the problem is equivalent to  (n^2-n+1)(n^2+(2k-1)n+(k^2-k+1)) = m^2 with {n,k} in the positive integers. 

But there are no such solutions:

{{k == -5, m == 7, n == 3}, {k == -3, m == 3, n == 2}, {k == -1, m == 1, n == 1}, {k == 0, m == 1, n == 0}, {k == 0, m == 1, n == 1}, {k == 0, m == 3, n == -1}, {k == 0, m == 3, n == 2}, {k == 0, m == 7, n == -2}, {k == 0, m == 7, n == 3}, {k == 1, m == 1, n == 0}, {k == 3, m == 3, n == -1}, {k == 5, m == 7, n == -2}}

Edited on September 27, 2020, 9:33 am
  Posted by broll on 2020-09-27 09:31:56

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