accum += Math.pow(values[i], 2.0);
I think Math.pow should not be used for small integer exponents, in particular 2,3 and perhaps 5. You could do this in FORTRAN or other languages with a built-in power operator, because the compiler will optimize it, but languages without that usually rewrite pos(x,y) as exp(y*log(x)). This is bad for precision, in particular if x is very small, and somewhat bad for performance (taking time of one multiplication and two divisions, typically costing 3 to 20 times as much as a multiplication each). I'm not sure how Java handles it, but the loss of precision may be responsible for the unexpected negative values.
I'd recommend to replace pow(x,2) by x*x, pow(x,3) by x*x*x and pow(x,4) by a=x*x, y=a*a. It doesn't matter much whether x is an expression because the compiler will take care of it and eliminate redundant computation, however code may be more redable if a temporary variable is used. If x is an integer, an explicit cast to double may be necessary to avoid integer overflow.
Math.pow should only be used if the exponent is larger, not an integer, or not a constant.
J.Pietschmann
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