# Reduce P = A(ATA)-1AT to P = BBT

1. Aug 14, 2009

### squenshl

How do I reduce P = A(ATA)-1AT to P = BBT whenever the column vectors of A form an orthonormal set.

2. Aug 14, 2009

### Peeter

Re: Proof

If the columns are an orthonormal set, then A^T A == I (where I has rank of the number of columns). The proof is mainly writing out that product in block form.