[Bug #902760] Testsuite fails with ARPACK 3.6 (#1107)

[Andreas Tille]

Hi,

just forwarding the suspicion of igraph upstream that the problem is
caused by arpack.  I'm tempted to reassign bug #902760 but I'd like
to hear your opinion first.

Kind regards

     Andreas.

----- Forwarded message from Tamás Nepusz <notifications at github.com> -----

Date: Thu, 26 Jul 2018 11:19:03 -0700
From: Tamás Nepusz <notifications at github.com>
To: igraph/igraph <igraph at noreply.github.com>
Cc: Andreas Tille <tille at debian.org>, Author <author at noreply.github.com>
Subject: Re: [igraph/igraph] Testsuite fails with ARPACK 3.6 (#1107)

Okay, I am pretty certain that this is an inherent issue with ARPACK 3.6; the same problem can also be triggered in GNU Octave (4.4.0) when linked with ARPACK 3.6 (I tried it on a Mac with Homebrew). The code below produces the PageRank matrix of the graph from the igraph test case:

```
octave:1> m = ones(11, 11) * 0.15 / 11;
octave:2> m(1, 2:11) += 0.85;
octave:3> m(2:11, 1) = (1 - m(1, 1)) / 10;
```

and it blows up exactly the same way:

```
octave:4> [v, lambda] = eigs(m, 1)
error: __eigs__: eigs: error -9999 in dnaupd
error: called from
    eigs at line 285 column 18
```

I'll file a bug report to `arpack-ng`.

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