Hello

Can you please address the following question?

I would like to implement Plot3D on a result of an operator. Specifically, please find below the following code, which consists of the following:

• define a Laplacian operator in 2D

• calculate the result of applying it to the specified function

• apply Plot3D

      oper = (D[#1, x, x] + D[#1, y, y]) &;
      f[x, y] == oper[Sin[x] Sin[y]];
      Plot3D[oper[Sin[x] Sin[y]], {x, 0, 10}, {y, 0,10}]
  

  The code above fails to plot. However, the following code succeeds

      oper = (D[#1, x, x] + D[#1, y, y]) &;
      f[x, y] == oper[Sin[x] Sin[y]];
      Plot3D[oper[Sin[x] Sin[y]] /. {x -> k, y -> l}, {k, 0, 10}, {l, 0,10}]
  

Can you please indicate why avoiding replacement doesn't work?