# Bug detected in Arpack

**URL:** <https://discourse.julialang.org/t/bug-detected-in-arpack/55334>\
**Category:** General Usage\
**Created:** [February 15, 2021, 4:32pm UTC](https://discourse.julialang.org/t/bug-detected-in-arpack/55334 "2021-02-15T16:32:31Z")\
**Posts on this page:** 7\
**Page:** 1

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**Author:** ![iamsuddhasattwa](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/iamsuddhasattwa/32/7441_2.png) [@iamsuddhasattwa](https://discourse.julialang.org/u/iamsuddhasattwa)\
**Post date:** [February 15, 2021, 4:32pm UTC](https://discourse.julialang.org/t/bug-detected-in-arpack/55334/1 "2021-02-15T16:32:31Z")

</div>

I can say with certainty that the **Arpack.eigs** has a very serious bug. I am running the following simple code to find the 4 smallest eigenvalues and eigenvectors of a 16X16 symmetric matrix. The file xxx.txt is uploaded [here](https://drive.google.com/file/d/1xFMU5ILqBA98udi86m_kNyro4xCjnBP1/view?usp=sharing).

```julia
using Serialization;
using Arpack;

K = deserialize("xxx.txt");
display(K);
k=4;
(vals, vecs) = Arpack.eigs(K; nev = k, which=:SM);

```

This is the output I get. You will notice that the matrix is pretty harmless.

```julia
16×16 Array{Float64,2}:
  0.659233 -0.195475 -0.0260898 0.0 -0.0917547 -0.234595 0.0 -0.14918 0.0 -0.02051 0.0 0.0 -0.031589 0.0 0.0 0.0
 -0.195475 0.61303 -0.141618 -0.0249161 -0.140206 -0.0322586 0.0 0.0 0.0 -0.11133 0.0 0.0 -0.014688 0.0 -0.0718231 0.0
 -0.0260898 -0.141618 0.853604 0.0 -0.130495 -0.0147248 0.0 0.0 0.0 -0.14347 -0.0275495 0.0 0.0 -0.0106849 0.0 -0.000746843
  0.0 -0.0249161 0.0 0.789886 0.0 0.0 -0.223093 0.0 0.0 0.0 0.0 -0.0826823 0.0 -0.232067 -0.0259092 0.0
 -0.0917547 -0.140206 -0.130495 0.0 0.666155 -0.0479611 0.0 -0.182288 0.0 -0.102586 0.0 0.0 0.0 0.0 0.0 0.0
 -0.234595 -0.0322586 -0.0147248 0.0 -0.0479611 0.714565 0.0 -0.155445 0.0 -0.025811 -0.0136642 0.0 -0.0280463 0.0 -0.0307591 -0.000370425
  0.0 0.0 0.0 -0.223093 0.0 0.0 0.75254 0.0 0.0 0.0 0.0 -0.091713 0.0 -0.257414 0.0 0.0
 -0.14918 0.0 0.0 0.0 -0.182288 -0.155445 0.0 0.687753 -0.124747 -0.0093745 -0.0215335 -0.0798659 -0.106118 0.0 -0.0608073 -0.0740178
  0.0 0.0 0.0 0.0 0.0 0.0 0.0 -0.124747 0.731166 -0.062372 -0.0831172 -0.107406 -0.159367 0.0 -0.0885275 -0.118728
 -0.02051 -0.11133 -0.14347 0.0 -0.102586 -0.025811 0.0 -0.0093745 -0.062372 0.748047 -0.185609 0.0 -0.0142867 0.0 0.0 -0.0705392
  0.0 0.0 -0.0275495 0.0 0.0 -0.0136642 0.0 -0.0215335 -0.0831172 -0.185609 0.687992 -0.0446934 -0.0328169 0.0 0.0 -0.210717
  0.0 0.0 0.0 -0.0826823 0.0 0.0 -0.091713 -0.0798659 -0.107406 0.0 -0.0446934 0.797492 -0.0933103 -0.0954023 -0.0550552 -0.168394
 -0.031589 -0.014688 0.0 0.0 0.0 -0.0280463 0.0 -0.106118 -0.159367 -0.0142867 -0.0328169 -0.0933103 0.847823 0.0 -0.0769094 -0.0866605
  0.0 0.0 -0.0106849 -0.232067 0.0 0.0 -0.257414 0.0 0.0 0.0 0.0 -0.0954023 0.0 0.449437 0.0 0.0
  0.0 -0.0718231 0.0 -0.0259092 0.0 -0.0307591 0.0 -0.0608073 -0.0885275 0.0 0.0 -0.0550552 -0.0769094 0.0 0.696568 -0.037958
  0.0 0.0 -0.000746843 0.0 0.0 -0.000370425 0.0 -0.0740178 -0.118728 -0.0705392 -0.210717 -0.168394 -0.0866605 0.0 -0.037958 0.660025
 1ERROR: LoadError: SingularException(16)
Stacktrace:
 [1] checknonsingular at /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.5/LinearAlgebra/src/factorization.jl:19 [inlined]
 [2] checknonsingular at /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.5/LinearAlgebra/src/factorization.jl:21 [inlined]
 [3] lu!(::Array{Float64,2}, ::Val{true}; check::Bool) at /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.5/LinearAlgebra/src/lu.jl:85
 [4] #lu#136 at /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.5/LinearAlgebra/src/lu.jl:273 [inlined]
 [5] lu at /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.5/LinearAlgebra/src/lu.jl:272 [inlined] (repeats 2 times)
 [6] factorize(::Array{Float64,2}) at /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.5/LinearAlgebra/src/dense.jl:1279
 [7] _eigs(::Array{Float64,2}, ::UniformScaling{Bool}; nev::Int64, ncv::Int64, which::Symbol, tol::Float64, maxiter::Int64, sigma::Nothing, v0::Array{Float64,1}, ritzvec::Bool) at /home/nodi/.julia/packages/Arpack/o35I5/src/Arpack.jl:164
 [8] #eigs#10 at /home/nodi/.julia/packages/Arpack/o35I5/src/Arpack.jl:46 [inlined]
 [9] #eigs#9 at /home/nodi/.julia/packages/Arpack/o35I5/src/Arpack.jl:45 [inlined]
 [10] top-level scope at /home/nodi/Documents/Julia/t.jl:10
 [11] include(::String) at ./client.jl:457
 [12] top-level scope at REPL[13]:1
in expression starting at /home/nodi/Documents/Julia/t.jl:8

```

I **urge the Julia developers to give this problem some serious consideration**. A language meant to do high performance computing should be able to find 4 eigenvalues of a 16X16 symmetric matrix. I could do it myself on pen and paper.

---

<div class="post-metadata">

**Author:** ![fgerick](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/fgerick/32/13228_2.png) [@fgerick](https://discourse.julialang.org/u/fgerick)\
**Post date:** [February 15, 2021, 5:28pm UTC](https://discourse.julialang.org/t/bug-detected-in-arpack/55334/2 "2021-02-15T17:28:31Z")

</div>

Looks like people are already aware of this bug: [eigs with singular matrix · Issue #87 · JuliaLinearAlgebra/Arpack.jl · GitHub](https://github.com/JuliaLinearAlgebra/Arpack.jl/issues/87)

and if you’re not willing to use pen and paper, you can also use `eigen` or [GitHub - JuliaLinearAlgebra/ArnoldiMethod.jl: Implicitly Restarted Arnoldi Method, natively in Julia](https://github.com/haampie/ArnoldiMethod.jl)

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<div class="post-metadata">

**Author:** ![iamsuddhasattwa](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/iamsuddhasattwa/32/7441_2.png) [@iamsuddhasattwa](https://discourse.julialang.org/u/iamsuddhasattwa)\
**Post date:** [February 15, 2021, 5:46pm UTC](https://discourse.julialang.org/t/bug-detected-in-arpack/55334/3 "2021-02-15T17:46:42Z")

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Dear @fgerick thank you very much for the links. I will pursue them.

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**Author:** ![Tamas\_Papp](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tamas_papp/32/25949_2.png) [@Tamas\_Papp](https://discourse.julialang.org/u/Tamas_Papp)\
**Post date:** [February 16, 2021, 9:52am UTC](https://discourse.julialang.org/t/bug-detected-in-arpack/55334/4 "2021-02-16T09:52:59Z")

</div>

> [@iamsuddhasattwa](#):
>
> A language meant to do high performance computing should be able to find 4 eigenvalues of a 16X16 symmetric matrix.

I am wondering if you are aware of

```julia
using LinearAlgebra
(vals, vecs) = eigen(Symmetric(K))

```

> [@iamsuddhasattwa](#):
>
> I could do it myself on pen and paper.

That’s very clever of you! Note, however, that this a _bug_ in a _package_ that is not part of the language _per se_. Bugs happen, and that does not reflect on what Julia is or isn’t able to do. Also, while I am as much of a fan as drama as the next person, it is unlikely to lead to the timelier resolution of bugs.

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<div class="post-metadata">

**Author:** ![iamsuddhasattwa](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/iamsuddhasattwa/32/7441_2.png) [@iamsuddhasattwa](https://discourse.julialang.org/u/iamsuddhasattwa)\
**Post date:** [February 16, 2021, 1:33pm UTC](https://discourse.julialang.org/t/bug-detected-in-arpack/55334/5 "2021-02-16T13:33:32Z")

</div>

> [@Tamas\_Papp](#):
>
> … that this a _bug_ in a _package_ that is not part of the language …

On the contrary it is very much a part of the language, and

> [@Tamas\_Papp](#):
>
> … Also, while I am as much of a fan as drama as the next person…

keep it up

ভাই আরেকটা কথা বলি আপনাকে। ভুল স্বীকার করতে না শিখলে উন্নতির পথ বন্ধ হয়ে যায়।

---

<div class="post-metadata">

**Author:** ![oxinabox](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/oxinabox/32/206603_2.png) [@oxinabox](https://discourse.julialang.org/u/oxinabox)\
**Post date:** [February 16, 2021, 2:24pm UTC](https://discourse.julialang.org/t/bug-detected-in-arpack/55334/6 "2021-02-16T14:24:19Z")

</div>

> [@iamsuddhasattwa](#):
>
> On the contrary it is very much a part of the language, and

I can assure you. ARPACK is a package.  
The [list of standard libraries is here](https://github.com/JuliaLang/julia/tree/master/stdlib), even for ones in seperate repos (they are listed at the bottom).

* * *

All software has bugs. Bugs are intenvitable.  
Obviously everyone tried to write bug free code, and has tests trying to prove it.  
but it is ineviatable that things slip though.  
You can look at any nontrivial project in any language.  
Even CPUs are regularly found to have bugs (see e.g. a log of the recent vulnerabilities, but even simpler things like calculators have been found to give incorrect results)  
This is why there are issue trackers.

On that note: please report bugs on the issue trackers; not Discourse.  
Maintainers don’t nescecarily check Discourse.  
and when you maintain dozens of packages trying to track what is going on in Discourse is just not reasonable.

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<div class="post-metadata">

**Author:** ![iamsuddhasattwa](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/iamsuddhasattwa/32/7441_2.png) [@iamsuddhasattwa](https://discourse.julialang.org/u/iamsuddhasattwa)\
**Post date:** [February 24, 2021, 9:23pm UTC](https://discourse.julialang.org/t/bug-detected-in-arpack/55334/7 "2021-02-24T21:23:40Z")

</div>

Thank you for the link.