Rcook was right. I provided an additional row (test case) and that generated the same answer as from wolfram/alpha.
D: 0.24999999999999822 D: 3.4500000000000033 D: 0.24999999999999914
Or 0.25x^2 + 3.45x + 0.25
final OLSMultipleLinearRegression regression2 = new OLSMultipleLinearRegression();
double[] y = {
4,
8,
13,
18
};
double[][] x2 =
{
{ 1, 1, 1 },
{ 1, 2, 4 },
{ 1, 3, 9 },
{ 1, 4, 16 },
};
regression2.newSampleData(y, x2);
regression2.setNoIntercept(true);
regression2.newSampleData(y, x2);
double[] beta = regression2.estimateRegressionParameters();
for (double d : beta) {
System.out.println("D: " + d);
}