I want to select some certain terms from a long expression. The expression contains parts like
I hope to get all terms with the form of
where a, b, c, d, can be arbitrary integers. I tried
Select[expr,MemberQ[#,k[a_, b_]+k[c_, d_]]&]
but it didn't work. Is there a way to realized this?
In addition, I also want to use
FullSimplify[expr]/.k[a_, b_] :> k[b, a]
to simplify the expression but when I apply it to the expression, nothing is simplified. Can someone help me with this? Thanks.