cleanF15a := 81*x^4 + 16*y^4 - 648*z^4 + 72*x^2*y^2 - 648 * x^2 - 288*y^2 + 1296;
noiseF15a := `*`(1/100, z^4 + 2*x^2*z^2  + y^2*z^2 - 7*z^2);