# Does gradient descent converge to a minimum-norm solution in least-squares problems?

Consider running gradient descent (GD) on the following optimization problem:

$$\arg\min_{\mathbf x \in \mathbb R^n} \| A\mathbf x-\mathbf b \|_2^2$$

where $$\mathbf b$$ lies in the column space of $$A$$, and the columns of $$A$$ are not linearly independent. Is it true that GD would find a solution with minimum norm? I saw some articles (e.g., 1705.09280) that indicated so, but I couldn't find a proof, searching on the internet for a while.

Can someone confirm or refute it? And if it's true, a proof or a reference to the proof would be much appreciated!

EDITS 2019/11/27:

Thanks to littleO's answer, apparently the answer to this question is no in general. However, I'm still curious about the following:

Follow-up Question: Are there some constraints under which the answer is yes? Is it true that, as Clement C. suggested, if we initialize $$\mathbf x$$ in the range of $$A^\top$$, then GD finds the minimum-norm solution? Is this a sufficient condition or is it also necessary?

It appears to me that the answer is yes, if and only if we initialize $$\mathbf x$$ in the range of $$A^\top$$.

I'll list my arguments below and would appreciate it if someone would confirm it or point out where I'm mistaken.

My arguments: Let $$f(\mathbf x)= \| A\mathbf x-\mathbf b \|_2^2$$. Then $$\nabla_{\mathbf x}f(\mathbf x) = 2A^\top(A\mathbf x - \mathbf b),$$ and GD iterates as follows: $$\mathbf x^{(t+1)}=\mathbf x^{(t)}-\eta \nabla_{\mathbf x}f(\mathbf x^{(t)})$$. Note that all GD updates are in the range of $$A^\top$$. Hence we may write $$\mathbf x^{(t)}=\mathbf x^{(0)}+A^\top \mathbf u$$ for some vector $$\mathbf u$$.

Sufficiency: Suppose $$\mathbf x^{(0)}$$ is also in the range of $$A^\top$$, i.e. $$\mathbf x^{(0)}=A^\top \mathbf v$$. Then $$\mathbf x^{(t)}=A^\top (\mathbf v+\mathbf u).$$ Since $$f(\mathbf x)$$ is convex, we know that GD will converge to a global minimum ($$0$$) if the step size is small enough. Denote this by $$\mathbf x^{(t)} \to \mathbf x^* = A^\top \mathbf u^*$$. Hence $$A\mathbf x^*-\mathbf b=AA^\top \mathbf u^*-\mathbf b=\mathbf 0$$, so $$\mathbf u^*=(AA^\top)^{-1}\mathbf b$$ (assuming $$A$$ is full rank), and $$\mathbf x^*=A^\top (AA^\top)^{-1}\mathbf b$$, which is the well-known minimum norm solution. (If $$A$$ is not full (row) rank, we can delete some redundant rows.)

Necessity: Now suppose $$\mathbf x^{(0)} \notin \mathrm{range}(A^\top)$$, and $$\mathbf x^{(t)} \to \mathbf x^*$$. We necessarily have $$\mathbf x^* = A^\top \mathbf u^* + \mathbf x^{(0)}$$ for some $$\mathbf u^*$$. However, clearly $$\mathbf x^*\notin \mathrm{range}(A^\top)$$, so it cannot possibly be the (unique) minimum norm solution, $$A^\top (AA^\top)^{-1}\mathbf b$$.

• This is true if you initialize at $0$, see e.g., the last two of these slides. (IIRC, this is true for most initializations as well, as long as it is in the span of the matrix) Nov 26, 2019 at 4:05
• @ClementC. This is very helpful. Thanks a lot! Nov 26, 2019 at 4:11
• You're welcome! Nov 26, 2019 at 4:14
• @MichaelGrant I was referring to the 2nd sentence of the "Introduction" Section, i.e. "For example, using gradient descent to optimize an unregularized, underdetermined least squares problem would yield the minimum Euclidean norm solution..." Nov 27, 2019 at 9:22
• There's a $\frac12$ missing in your objective function. Jan 6, 2020 at 22:29

From the paper  in question:

When optimizing underdetermined problems with multiple global minima, the choice of optimization algorithm can play a crucial role in biasing us toward a specific global minima, even though this bias is not explicitly specified in the objective or problem formulation. For example, using gradient descent to optimize an unregularized, underdetermined least squares problem would yield the minimum Euclidean norm solution, while using coordinate descent or preconditioned gradient descent might yield a different solution. Such implicit bias, which can also be viewed as a form of regularization, can play an important role in learning.

Given fat matrix $$\mathrm A \in \mathbb R^{m \times n}$$ ($$m < n$$) and vector $$\mathrm b \in \mathbb R^m$$, consider the following linear system in $$\mathrm x \in \mathbb R^n$$

$$\rm A x = b$$

where $$\rm A$$ has full row rank. Let the singular value decomposition (SVD) of $$\rm A$$ be as follows

$$\mathrm A = \mathrm U \Sigma \mathrm V^\top = \mathrm U \begin{bmatrix} \Sigma_1 & \mathrm O \end{bmatrix} \begin{bmatrix} \mathrm V_1^\top \\ \mathrm V_2^\top \end{bmatrix} = \mathrm U \Sigma_1 \mathrm V_1^\top$$

The least-norm solution of $$\rm A x = b$$ is given by

$$\mathrm x_{\text{LN}} := \mathrm A^\top \left( \mathrm A \mathrm A^\top \right)^{-1} \mathrm b = \cdots = \mathrm V_1 \Sigma_1^{-1} \mathrm U^\top \mathrm b$$

where the inverse of $$\mathrm A \mathrm A^\top$$ exists because $$\rm A$$ has full row rank.

Let cost function $$f : \mathbb R^n \to \mathbb R$$ be defined by

$$f (\mathrm x) := \frac12 \left\| \rm{A x - b} \right\|_2^2$$

$$\nabla f (\mathrm x) = \rm A^\top \left( A x - b \right)$$

Using gradient descent with step $$\mu > 0$$,

\begin{aligned} {\rm x}_{k+1} &= {\rm x}_k - \mu \nabla f ({\rm x}_k)\\ &= \left( {\rm I} - \mu {\rm A^\top A} \right) {\rm x}_k + \mu {\rm A^\top b}\end{aligned}

Hence,

$${\rm x}_k = \left( {\rm I} - \mu {\rm A^\top A} \right)^k {\rm x}_0 + \mu \sum_{\ell = 0}^{k-1} \left( {\rm I} - \mu {\rm A^\top A} \right)^{\ell} {\rm A^\top b}$$

Letting $$\rm y := V^\top x$$, we rewrite

\begin{aligned} {\rm y}_k &= \left( {\rm I} - \mu \Sigma^\top \Sigma \right)^k {\rm y}_0 + \mu \sum_{\ell = 0}^{k-1} \left( {\rm I} - \mu \Sigma^\top \Sigma \right)^{\ell} \Sigma^\top {\rm U^\top b}\\ &= \begin{bmatrix} \left( {\rm I} - \mu \Sigma_1^2 \right)^k & \mathrm O\\ \mathrm O & \mathrm I\end{bmatrix} {\rm y}_0 + \mu \sum_{\ell = 0}^{k-1} \begin{bmatrix} \left( {\rm I} - \mu \Sigma_1^2 \right)^{\ell} & \mathrm O\\ \mathrm O & \mathrm I\end{bmatrix} \begin{bmatrix} \Sigma_1\\ \mathrm O \end{bmatrix} {\rm U^\top b}\\ &= \begin{bmatrix} \left( {\rm I} - \mu \Sigma_1^2 \right)^k & \mathrm O\\ \mathrm O & \mathrm I\end{bmatrix} {\rm y}_0 + \mu \sum_{\ell = 0}^{k-1} \begin{bmatrix} \left( {\rm I} - \mu \Sigma_1^2 \right)^{\ell} \Sigma_1\\ \mathrm O\end{bmatrix} {\rm U^\top b} \end{aligned}

Choosing $$\mu > 0$$ such that all eigenvalues of $${\rm I} - \mu \Sigma_1^2$$ are strictly inside the unit circle, then $${\rm y}_k \to {\rm y}_{\infty}$$, where

$${\rm y}_{\infty} = \begin{bmatrix} \mathrm O & \mathrm O\\ \mathrm O & \mathrm I\end{bmatrix} {\rm y}_0 + \mu \sum_{\ell = 0}^{\infty} \begin{bmatrix} \left( {\rm I} - \mu \Sigma_1^2 \right)^{\ell} \Sigma_1\\ \mathrm O\end{bmatrix} {\rm U^\top b}$$

where

$$\mu \sum_{\ell = 0}^{\infty} \left( {\rm I} - \mu \Sigma_1^2 \right)^{\ell} \Sigma_1 = \mu \left( {\rm I} - {\rm I} + \mu \Sigma_1^2 \right)^{-1} \Sigma_1 = \Sigma_1^{-1}$$

and, thus,

$${\rm y}_{\infty} = \begin{bmatrix} \mathrm O & \mathrm O\\ \mathrm O & \mathrm I\end{bmatrix} {\rm y}_0 + \begin{bmatrix} \Sigma_1^{-1} \\ \mathrm O\end{bmatrix} {\rm U^\top b}$$

Since $$\rm x := V y$$,

$$\boxed{ \,\\\quad {\rm x}_{\infty} = {\rm V}_2 {\rm V}_2^\top {\rm x}_0 + \underbrace{{\rm V}_1 \Sigma_1^{-1}{\rm U^\top b}}_{= \mathrm x_{\text{LN}}} \quad\\}$$

Therefore, we conclude that if $${\rm x}_0$$ is orthogonal to the null space of $$\rm A$$, then gradient descent will converge to the least-norm solution.

 Suriya Gunasekar, Blake Woodworth, Srinadh Bhojanapalli, Behnam Neyshabur, Nathan Srebro, Implicit Regularization in Matrix Factorization, May 2017.

• I appreciated the detailed proof showing the convergence of gradient descent. Your proof also implied that $x_0\in \mathrm{Null}(A)^\perp$ is also necessary for statement to hold. Since $\mathrm{Null}(A)^\perp = \mathrm{Row}(A) = \mathrm{Range}(A^\top)$, the conclusion is confirmed. Jan 7, 2020 at 1:44
• Thank you for the suggestion. I'd think that the authors probably knew the restriction, but simply omitted it in the introduction section, perhaps because this was supposedly known to folks who have been working on the machine learning generalization theory and inductive bias of learning algorithms. (But not to me. I'm new here.) Jan 8, 2020 at 8:39

If you initialize gradient descent with a point $$x_0$$ which is a minimizer of the objective function but not a least norm minimizer, then the gradient descent iteration will have $$x_k = x_0$$ for all $$k \geq 0$$. We won't move anywhere. So gradient descent does not necessarily converge to a least norm solution.

• Indeed. Thanks for pointing out this obvious fact. So this is not true in general. But I'm still curious about the validity of the claim in the question, e.g. with random small initialization. I'll edit the question. Nov 26, 2019 at 4:05
• Perhaps it is true everywhere but the optimal set. Nov 28, 2019 at 14:42
• @MichaelGrant Perhaps, but I doubt it. There is a whole business of literature devoted to solving "best approximation" problems -- problems which, on top of minimizing an objective and satisfying constraints, we need to find the solution closest to a prescribed point. In this problem, we want to find the closest solution to $0$. My suspicion is that, if this sort of thing was always done by default with gradient descent, then there wouldn't be an entire field devoted to the problem.
– Zim
Oct 14, 2020 at 12:57
• But gradient descent is an unconstrained algorithm, it is first order, and is not appropriate for all situations. I don't think the field you mention would die even if my supposition were true. It might also be true only in specific scenarios (such as, smallstepsize) where one is likely not to want to work in practice. nevertheless, your skepticism is reasonable. Oct 14, 2020 at 13:02