I was going about finding the pattern of integers \(a\) such that \(x+a=x^2\) has at least one rational solution. Using the quadratic formula, the value boiled down to \(\frac{1\pm\sqrt{1+4a}}{2}\). Thus, this occurs when \(4a+1\) is the square of an integer.
I knew that odd squares can be expressed as \(4a+1\), which is not very hard to prove.
Consider \(y^2-1\), for some odd integer \(y\). Now, this can be factorized into \((y+1)(y-1)\). Now, \(2|y+1,2|y-1\), since \(y\) is odd. Thus, \(4|y^2-1\), or any odd \(y^2\) can be expressed as \(4a+1\).
Now, I was making a table of odd integers and their squares to find any patterns. Only after \(25\), did I realise a pattern.
For \(49\), \(a=12\).
For \(81\), \(a=20\).
For \(121\), \(a=30\).
For \(169\), \(a=42\).
I realised a pattern, in which the difference between consecutive values of \(a\) are in an arithmetic progression with difference of \(2\).
I tested my theory, and found it to comply, as for \(225\), \(a=56\), which I predicted.
Is their a proof for this theory? Did I stumble upon some intricate pattern?
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Nanayaranaraknas Vahdam
3 years, 10 months ago
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Top NewestFrom the above question, \(a = x^{2} - x = x(x-1)\). The examples you're interested in positive integers, that will be the common pattern of \(2,6,12,20,30,42,...\)
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It's pretty easy to see that the difference of two consecutive a's is 2 by using that y must be odd and writing it as 2k-1.
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