The number of distinct simple graph with upto three nodes is

In a compact single dimensional array representation for lower triangular matrices (i.e., all the elements above the diagonal are zero) of size $$n$$ $$x$$ $$n$$, non-zero elements (i.e., elements of the lower triangle) of each row are stored one after another, starting from the first row, the index of the $${\left( {i,\,j} \right)^{th}}$$ element of the lower triangular matrix in this new representation is

The inverse of the matrix $$\left[ {\matrix{ 1 & 0 & 1 \cr { - 1} & 1 & 1 \cr 0 & 1 & 0 \cr } } \right]$$ is

The rank of the matrix $$\left[ {\matrix{ 0 & 0 & { - 3} \cr 9 & 3 & 5 \cr 3 & 1 & 1 \cr } } \right]$$ is