Wikipedia:Reference desk/Archives/Mathematics/2014 September 30

= September 30 =

Quadratic expression evaluates to perfect square
For example, 2x^2 - 238x + 14161. Is there any way, apart from just substituting a succession of x values, to determine which integers will give the square of an integer? If my calculation is correct, the first eight positive ones are 20, 35, 39, 51, 68, 80, 84 and 99, which give the squares of 101, 91, 89, 85, 85, 89, 91 and 101. I see no obvious pattern beyond the symmetry about x=59.5 for a minimum.←86.146.61.61 (talk) 16:42, 30 September 2014 (UTC)


 * Google quadratic Diophantine equation, or use http://alpertron.com.ar/QUAD.HTM. PrimeHunter (talk) 16:53, 30 September 2014 (UTC)
 * Note for the QUAD, you are looking for x and y that fit 2x^2 - 238x + 14161 =y^2, move the y^2 to the other side, so the entries on the left are 2, 0, -1, -238, 0 and 14161.Naraht (talk) 17:37, 30 September 2014 (UTC)


 * Thanks. I've realised that 2x^2 - 238x + 14161 = y^2 reduces to x^2 + (119-x)^2 = y^2, so a search of, or parameterisation of, Pythagorean triples will suffice in this case.←86.146.61.61 (talk) 19:10, 30 September 2014 (UTC)

Automatic calculation with Excel
I don't want an explanation of how to do it - just even give me the name of the concept\function and I'll read about it.

I need to get to know the Automatic calculation option of statistical data with Excel (for example, for getting a Standard deviation). Right now, I can only write the data myself (e.g Multiply\Power every single piece) and I ought to automate this process. Thank you for your help and guidance. Ben-Natan (talk) 17:14, 30 September 2014 (UTC)
 * This question more properly belongs in the computing desk, also I'm not quite sure what you're asking for, but I'll give it a try. First, there are two standard deviation estimates commonly used, one with N-1 and one with N. (If you know the formulas the you'll know what I'm talking about.) Excel has the N-1 version as a built in function STDEV and the N version as STDEVP. I think what you're trying to do is get Excel to include one or both of these in the status bar along with min, max, sum and average. There doesn't seem to be a way of doing that, at least with the version of Office I'm using which is fairly recent. Maybe the next version will do it, but until then I think you'll have to make do with the STDEV or STDEVP. --RDBury (talk) 18:53, 30 September 2014 (UTC)
 * A more basic goal: I want that if I have 3 columns: X, f, and f*x, Than, Excel would multiply x&f for me. I want to achieve this goal for saving time. Ben-Natan (talk) 20:09, 30 September 2014 (UTC)
 * Again, your question is not very clear, but if I understand correctly then this is a very basic use of Excel. If X and f are in columns A and B, with the values starting on row 2, then you would enter =A2*B2 (a Formula in Excel terminology) in cell C2 and then copy it down as far as you need it, e.g. by dragging the "handle" (small square) that appears at the bottom right of the cell. Apologies if I've misinterpreted, but if this is new to you then you really need to do some basic tutorials before going much further. AndrewWTaylor (talk) 20:17, 30 September 2014 (UTC)
 * This is sometimes called "replication" of the formula.   D b f i r s   22:17, 30 September 2014 (UTC)
 * Andrew, I don't know what wasn't clear to you in the question, but anyway The third column (f*X) should just contain the multiplications of X(Column A), and f(Column B). Is what you've described is the way to achieve this very goal? Does this process have a clear name that I could look for some Youtube videos about it? thank you again. Ben-Natan (talk) 04:21, 1 October 2014 (UTC)
 * Do you mean just the "Insert >> Function" feature in Excel? Any basic tutorial (or the inbuilt help) will explain this.    D b f i r s   08:15, 1 October 2014 (UTC)
 * Yes, the method I described will achieve what you want. The "clear name" for the key concept is formula, and it's absolutely basic in using Excel.  This page (for example) might give you a start; no doubt there are plenty of others, e.g. this YouTube video.  AndrewWTaylor (talk) 15:13, 1 October 2014 (UTC)