hello

I am new to the forum so I don't know if I am posting in the right section

I want to prove the following equality in index notation :

S[i,k]S[k,j]S[i,l]S[l,j]=1/2*S[i,j]S[i,j]S[k,l]S[k,l] .

S is a symmetric second-order tensor with property S[i,i] = 0 .

I have been struggling with this for a week ,any help is appreciated, thanks in advance