# Rank of matrix with diagonal 0, and others +-1

Let $$B$$ be a $$(n-1)×(n-1)$$ matrix such that: all elements on diagonal equal $$0$$; and all other either $$1$$ or $$\text{-}1$$.

Let $$A = \begin{bmatrix}B&(1,...,1)^T\\ (1,...,1)&1\end{bmatrix}$$, so $$A$$ be a $$n×n$$ derived from $$B$$ by adding a row and column with $$1$$.

What could be the rank of the matrix $$A$$ ?

• As established in this post, $B$ has a submatrix of rank at most $n-2$. Since this submatrix is in turn a submatrix of $A$, it follows that the rank of $A$ is $n-2$, $n-1$, or $n$. I suspect, however, that $A$ will generally be invertible (with odd determinant). – Omnomnomnom Apr 7 at 7:29
• If we replace the bottom-right entry with a $0$, we get a matrix that, when taken modulo $2$, has eigenvalues $n-1$ with multiplicity $1$ and $-1$ with multiplicity $n-1$. I have not found a way to leverage this fact. – Omnomnomnom Apr 7 at 7:52

$$A$$ is necessarily of full rank, since its determinant is non-zero. We can show that the determinant is non-zero by showing that it is necessarily an odd number.
Following Hans's idea here, showing that $$A$$ always has odd determinant is equivalent to showing that the number of permutations on $$n$$ objects that either have no fixed point or fix only the final entry is odd. If $$d_n$$ denotes the number of derangements, then we wish to show that $$d_n + d_{n-1}$$ is necessarily odd.
We note that $$d_n$$ satisfies the recurrence relation $$d_1 = 0, \quad d_n = n d_{n-1} + (-1)^n$$ so that $$d_n$$ is odd iff $$d_{n-1}$$ is even. It follows that $$d_n + d_{n-1}$$ is always the sum of an even and odd number, and is therefore odd.
• $d_{n-1}$ corresponds to permutations with fixed one element, bottom right entry. And $d_{n}$ corresponds to permutations without fixed elements ? – Ivan Apr 7 at 11:22
• Exactly ${}{}{}{}{}$ – Omnomnomnom Apr 7 at 11:49