Hello !

I am a new user of mathematica and i am struggling with the Taylor (Mclaren) series. I want to expand the following multi variable function with respect to "xsi0" and "rho0" up to O(1) :

y1[z_] := (R + Subscript[\[Rho], 0] E^(I \[Gamma] z )) Cos[
\!\(\*SuperscriptBox[\(k\), \('\)]\) (z - 
       Subscript[\[Xi], 0] E^(I \[Gamma] z ))];

So I do :

y1e[z_] := 
 Series[y1[z], {Subscript[\[Xi], 0], 0, 1}, {Subscript[\[Rho], 0], 0, 
     1}] // Normal // Expand
y1e[z]

Which gives the correct expansion but a mixed derivative term is also included (which is technically a second order term). I would like to get rid of it. Is there a clean way to do so ?