Computing log(0, 2) takes a long time

[cyrilbouvier]

The following commands run "instantly":

sage: log (0.0)
-infinity
sage: log (0.0, 2)
-infinity
sage: log (0)
-Infinity

But the command

sage: log (0, 2)

takes a very long time (at least a few minutes). Moreover, the python process needs to be killed, using Ctrl+C does not work to stop the computation.

I was able to reproduce the bug with Sage 9.0 (using Python3) and Sage 8.9 (using Python2). But i was not able to reproduce the bug with Sage 7.6.

Component: basic arithmetic

Keywords: log float integer

Author: Ben Livingston

Branch/Commit: e44937e

Reviewer: Frédéric Chapoton

Issue created by migration from https://trac.sagemath.org/ticket/29164

[mkoeppe]

comment:1

Batch modifying tickets that will likely not be ready for 9.1, based on a review of the ticket title, branch/review status, and last modification date.

[bmlivin]

comment:4

In sage.rings.integer.Integer.exact_log, this triggers a ValueError:

if mpz_sgn(self.value) <= 0 or mpz_sgn(_m.value) <= 0:
    raise ValueError("both self and m must be positive")

In your example, self.value is 0 and _m.value is 2. It seems to me like it would be better to return -infinity instead of raising this error if self.value is 0, but in a sense that is beside the point. What instead happens is that this ValueError is in a try-except in sage.functions.log.log:

    try:
        return args[0].log(args[1])
    except ValueError as ex:
        if repr(ex)[12:27] == "No discrete log":
            raise
        return logb(args[0], args[1])

Here, args[0] is 0 and args[1] is 2. Calling the function logb results in the same ValueError and the same try-except, and thus an infinite loop.

To avoid this, -infinity needs to be returned at some point. I think that point should be in sage.rings.integer.Integer.exact_log, immediately before raising the ValueError.

[bmlivin]

Branch: u/bmlivin/29164

[bmlivin]

Changed branch from u/bmlivin/29164 to u/gh-bmlivin/29164

[bmlivin]

Changed branch from u/gh-bmlivin/29164 to none

[bmlivin]

Branch: u/gh-bmlivin/computing_log_0__2__takes_a_long_time

[bmlivin]

Commit: fe8c387

[bmlivin]

New commits:

0f35da0	Fixed issue when taking log(0, n) for integer n
fe8c387	Added a doctest to sage.functions.log.log to check that issue 29164 is fixed

[fchapoton]

comment:11

in the doctest, please add the missing empty line after ::

+    
+    Check if :track:`29164` is fixed::
+        sage: log(0, 2)
+        -Infinity

[fchapoton]

comment:12

also use :trac: and not :track:

[sagetrac-git]

Changed commit from fe8c387 to e44937e

[sagetrac-git]

Branch pushed to git repo; I updated commit sha1. New commits:

e44937e

adding empty line and changing track to trac in doctest of sage.functions.log.log

[slel]

comment:14

Full names in "Authors" and "Reviewers" fields please.

[fchapoton]

Reviewer: Frédéric Chapoton

[fchapoton]

comment:16

Ben, please add your full real name in the author field here above.

[bmlivin]

Author: Ben Livingston

[fchapoton]

comment:18

ok, let it be then. Thanks

[vbraun]

Changed branch from u/gh-bmlivin/computing_log_0__2__takes_a_long_time to e44937e