# Solution of underdetermined systems using LAPACK

**URL:** <https://discourse.julialang.org/t/solution-of-underdetermined-systems-using-lapack/33000>\
**Category:** Performance\
**Created:** [January 5, 2020, 5:38pm UTC](https://discourse.julialang.org/t/solution-of-underdetermined-systems-using-lapack/33000 "2020-01-05T17:38:47Z")\
**Posts on this page:** 1\
**Showing post:** 4

<div class="post-metadata">

**Author:** ![stevengj](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/stevengj/32/71_2.png) [@stevengj](https://discourse.julialang.org/u/stevengj)\
**Post date:** [January 5, 2020, 7:10pm UTC](https://discourse.julialang.org/t/solution-of-underdetermined-systems-using-lapack/33000/4 "2020-01-05T19:10:14Z")

</div>

> [@lucas711642](#):
>
> I need to get any solution of a bunch of underdetermined systems `A*x = b` fastly and cheaply.

Just do `x = A \ b` — it gives you the minimum-norm solution by default when `A` has more columns than rows. If you need to solve a sequence of right-hand sides for the same `A`, use the pivoted QR factorization:

```julia
QR = qr(A, Val(true))
x = QR \ b

```

If you [search discourse for “minimum norm”](https://discourse.julialang.org/search?q=minimum%20norm) you will find more discussions of these topics.

---

_[View the full topic](https://discourse.julialang.org/t/solution-of-underdetermined-systems-using-lapack/33000)._