Guillermo,
@Sander Huisman has a great solution. Another alternative that works on any expression (not just ra[]).
mysq[x_Plus] := Map[mysq, x]
mysq[x_] := x^2
SetAttributes[mysq, Listable]
In[13]:= i = {a, b + c + d, c};
mysq[i]
Out[14]= {a^2, b^2 + c^2 + d^2, c^2}
Regards,
Neil
