public void ValidateDensityLn(int shape, double invScale, double x, double pdfln)
{
var n = new Erlang(shape, invScale);
AssertHelpers.AlmostEqualRelative(pdfln, n.DensityLn(x), 13);
AssertHelpers.AlmostEqualRelative(pdfln, Erlang.PDFLn(shape, invScale, x), 13);
}