There is a function Y ( a , b ,c ) which I am interested in. I have a computer program which can find the value of Y, therefore I know the value of Y for any a, b and c. My goal is to find an approximate formula of Y so I could calculate Y without using the computer program. I do now that a, b and c are interchangeable, for example : Y ( 1, 2, 3) = Y ( 2, 1, 3) = Y( 3, 2, 1) =... and so on. Moreover, I am only interested in the function Y if a + b + c =constant, otherwise the function Y ( a, b, c) is inaccurate. What should I do ?

Recently I had a similar problem, but the function I was interested in Z ( x ) had only one variable. I've written down a number of values Z, created a Z ( x ) plot and fitted a curve on it, which gave me a satisfying approximation. However, I'm not sure what to do with the function Y ( a, b, c ) since it has 3 variables.

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