# Lecture 8 Least-norm solutions of undetermined equations

EE263 Autumn 2007-08

Stephen Boyd

Lecture 8 Least-norm solutions of undetermined

equations

? least-norm solution of underdetermined equations ? minimum norm solutions via QR factorization ? derivation via Lagrange multipliers ? relation to regularized least-squares ? general norm minimization with equality constraints

8?1

Underdetermined linear equations

we consider y = Ax

where A Rm?n is fat (m < n), i.e.,

? there are more variables than equations ? x is underspecified, i.e., many choices of x lead to the same y

we'll assume that A is full rank (m), so for each y Rm, there is a solution set of all solutions has form

{ x | Ax = y } = { xp + z | z N (A) } where xp is any (`particular') solution, i.e., Axp = y

Least-norm solutions of undetermined equations

8?2

? z characterizes available choices in solution ? solution has dim N (A) = n - m `degrees of freedom' ? can choose z to satisfy other specs or optimize among solutions

Least-norm solutions of undetermined equations

8?3

Least-norm solution

one particular solution is xln = AT (AAT )-1y

(AAT is invertible since A full rank)

in fact, xln is the solution of y = Ax that minimizes x

i.e., xln is solution of optimization problem

minimize x subject to Ax = y

(with variable x Rn)

Least-norm solutions of undetermined equations

8?4

suppose Ax = y, so A(x - xln) = 0 and (x - xln)T xln = (x - xln)T AT (AAT )-1y = (A(x - xln))T (AAT )-1y =0

i.e., (x - xln) xln, so x 2 = xln + x - xln 2 = xln 2 + x - xln 2 xln 2

i.e., xln has smallest norm of any solution

Least-norm solutions of undetermined equations

8?5

{ x | Ax = y }

N (A) = { x | Ax = 0 } xln

? orthogonality condition: xln N (A)

? projection interpretation: xln is projection of 0 on solution set { x | Ax = y }

Least-norm solutions of undetermined equations

8?6

? A = AT (AAT )-1 is called the pseudo-inverse of full rank, fat A ? AT (AAT )-1 is a right inverse of A ? I - AT (AAT )-1A gives projection onto N (A)

cf. analogous formulas for full rank, skinny matrix A: ? A = (AT A)-1AT ? (AT A)-1AT is a left inverse of A ? A(AT A)-1AT gives projection onto R(A)

Least-norm solutions of undetermined equations

8?7

Least-norm solution via QR factorization

find QR factorization of AT , i.e., AT = QR, with ? Q Rn?m, QT Q = Im ? R Rm?m upper triangular, nonsingular

then ? xln = AT (AAT )-1y = QR-T y ? xln = R-T y

Least-norm solutions of undetermined equations

8?8

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