Solution
= Solution
Setting either argument to zero in the expansion and comparing coefficients gives
$$
\boxed{F^r=0,\qquad A=B=1,\qquad
D^r{}_{st}=E^r{}_{st}=0}.
$$
Thus, through quadratic order,
$$
\varphi^r(x,y)=x^r+y^r+C^r{}_{st}x^sy^t+O(3).
$$