A non-negative square matrix $A$ is ultrametric iff:
- $A(i,i)>\{A(i,k),A(k,i)\}\forall k,i$
- $A(i,j)\geq\min\{A(i,k),A(k,j)\}\forall i,j,k$
It is well-known that the inverse of non-negative ultrametric is always diagonally dominant.
Now assume that, we slightly relax the second constraint and change it to:
- If $A(i,j)>0$ then $A(i,j)\geq\min\{A(i,k),A(k,j)\} \forall i,j,k$
In words, the second constraint only holds for non-zero entries. Also for simplicity we can assume that the diagonal of $A$ is all $1$, and other entries are less than $1$.
My question is can we say any of the following about $A^{-1}$:
- $A^{-1}$ is diagonally dominant.
- $A^{-1}(i,i) > \sum_{k\neq i, A(i,k)>0} |A^{-1}(i,k)|$
Specially I think the second one should be true.
Any comments will be appreciated.