Suppose L∈Rn×n is a singular, lower triangular matrix. Is its psuedoinverse, L†∈Rn×n, also lower triangular? I have already proved by induction that the product of two lower triangular matrices is lower triangular, and I also proved that the inverse of a (non-singular) lower triangular matrix is lower triangular. I have shown by working out an example with lower triangular L∈R2×2 that the pseudoinverse, L†∈R2×2, does not have to be lower triangular, but I was wondering if there might be a better way to prove this than just by showing through an example in R2×2 that L† is not necessarily lower triangular.
Useful information:
The pseudoinverse of a matrix A satisfies the following properties:
1) (A†A)∗=A†A
2) (AA†)∗=AA†
3) AA†A=A
4) A†AA†=A†
Note: here I will be considering only the reals, so the conjugate transpose becomes just a transpose.
Attempt at Solution:
For L∈Rn×n, where n=1, it is easy to show that L† is lower triangular. Since L is singular, we have L=0, which is also lower triangular, by the definition of a lower triangular matrix. Then, properties 1) through 3) above are trivially satisfied by any L†∈R, but for property 4) to be satisfied, we need L†0L†=L†⟹L†=0. But, L† is lower triangular by the definition of a lower triangular matrix, so for n=1, the answer to the original question is "yes".
Now, consider the case of n=2. Let
L=(a0b˜L) for a and b∈R, and with ˜L a singular, lower triangular matrix ∈R1×1, as before in the example with n=1. So L is a singular, lower triangular matrix, and L=(a0b0). Let L†∈R2×2 be the pseudoinverse of L, and define L† as L†=(cde˜L†), with c,d,e∈R and with ˜L† being the psuedoinverse of ˜L as derived in the above example for n=1, so ˜L†=0 and so L†=(cde0).
Now, property 1) above requires ea=0. Property 2) requires ad=bc. Property 3) requires that (a=0 or ca+db=1) and (b=0 or ca+db=1). Property 4) requires e=0 and (c=0 or ca+db=1) and (d=0 or ca+db=1). I chose d≠0, which would then make L† not a lower triangular matrix. Then, with ca+db=1 and d≠0, I found b=1−cad. Further, from ad=bc, I found (substituting the previous expression for b), a=cc2+d2, which then gives b in terms of c and d as b=dc2+d2. My L and L† then become L=1c2+d2(c0d0), and L†=(cd00). L† satisfies all four of the properties above, but it is not lower triangular, so my answer to the original question is, in general, "no".
However, surely their is a smarter way to do this, in which I don't have to resort to doing all this algebra, isn't there? If so, how might I approach the proof? Thanks a lot!
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