Polyfitn函数是polyfit的扩展，允许用户实现多个独立变量的模型拟合。
Polyfitn is an extension of polyfit, allowing the user to create models with more than one independent variable.
允许用户自定义通用的模型，例如，具有常数项和二次项、但没有线性项的二次模型。
It also allows the user to specify a general model, for example, a quadratic model, with constant and quadratic terms, but no linear term.
例如，为了将余弦曲线中选择的点拟合为多项式模型，我们只需要偶数阶项。
For example, to fit a polynomial model to points selected from a cosine curve, we will only need the even ordered terms.
x = -2:.1:2;
y = cos(x);
p = polyfitn(x,y,‘constant x^2 x^4 x^6’);
p.Coefficients
ans =
[0.99996 -0.49968 0.041242 -0.0012079]
x = rand(100,1);
y = rand(100,1);
z = exp(x+y) + randn(100,1)/100;
p = polyfitn([x,y],z,3);
源码下载地址：
http://page5.dfpan.com/fs/7ldcaj32e2311239163/
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