# Substituting a function in the output

I have to calculate the derivatives of the function K defined in the code:

<< VectorAnalysis
K = δ[t]*Exp[-(r/L)]/r;
SetCoordinates[Cartesian[x, y, z]];

r = Sqrt[(x - x0)^2 + (y - y0)^2 + (z - z0)^2];
Hes = D[K, {{x, y, z}, 2}];
FullSimplify[der[[1]]]
FullSimplify[Hes[[1, 2]]]
FullSimplify[Hes[[1, 1]]]


This is part the output just for FullSimplify[der[[1]]]:

-((E^(-(Sqrt[(x - x0)^2 + (y - y0)^2 + (z - z0)^2]/ L)) (x - x0) (L +
Sqrt[(x - x0)^2 + (y - y0)^2 + (z - z0)^2]) δ[t])/( L ((x - x0)^2 + (y - y0)^2 +
(z - z0)^2)^(3/2)))


I would like the function K to appear in the result instead of that "mess": is it possible to do it?

-

I'm not sure what's your goal since you've expressed K and switched coordinates.
Simplify[(der /. r^2 -> rad^2), rad > 0] /. E^(-(rad/L)) -> newK rad/δ[t]
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