I have a polynomial in x and y. I want to keep all the terms with (combined) degree 4. Any pointers will be appreciated. Simple tricks for doing the same with univariate polynomial don't seem to work.
1 Answer
p = Total@Flatten@Table[ a[i, j] x^i y^j , {i, 0,5}, {j, 0, 5}]
Total@Flatten@
MapIndexed[
If[Total@#2 - 2 == 4 , x^(#2[[1]] - 1) y^(#2[[2]] - 1) #1, 0]&,
CoefficientList[p, {x, y}], {2}]
y^4 a[0, 4] + x y^3 a[1, 3] + x^2 y^2 a[2, 2] + x^3 y a[3, 1] + x^4 a[4, 0]
Obviously put <=4
in there to keep the lower terms..
t
.. $\endgroup$