Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

What should I assume when I see log?

  1. Sep 30, 2013 #1
    Today in a lecture in one of my engineering courses the professor said that mathematicians assume that when log is written the base is e. I had always learned to assume base 10, and simply write ln when I wanted the base as e.

    Is my professor correct or is that a bad habit to adapt?
  2. jcsd
  3. Sep 30, 2013 #2


    User Avatar
    Gold Member
    2016 Award

    I'm with you.
  4. Sep 30, 2013 #3


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    In math and physics log10 is rarely used, assume base e. I also use ln for base e.
  5. Sep 30, 2013 #4


    User Avatar
    Science Advisor
    Gold Member

    Context is the main determining factor. When doing analysis, assume e. When doing calculation, assume 10. Often to distinguish, base e is denoted by ln(x) rather than log(x).
  6. Sep 30, 2013 #5


    User Avatar
    Science Advisor
    Homework Helper

    That's a perfectly reasonable "rule", but don't expect everybody else to use it!

    Well, calculator buttons are often labeled log and ln, but computer programming languages tend to use log() for base e and log10() for ... um .... some other base, maybe 42? :biggrin:

    Just remember that one way to have a quiet life is use the same notation as your prof.

    And number theorists think ##\log_k x## means ##\ln \ln \dots\ \ln x##. (i.e take the natural log k times).
  7. Sep 30, 2013 #6
    In addition to mathematicians, who 99% of the time mean the natural log when they say "log", there are also computer scientists, who usually mean the log base 2.

    In most cases, it's either clear from context or doesn't matter.
  8. Oct 1, 2013 #7


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    Dearly Missed

    In certain professional fields, such as in some cases in fluid mechanics, established tradition uses log_10 constants in a few, handy formulae (I was in much surprize and consternation when I met them! The author had at least the minimal decency to write, explicitly, log_10, rather than log).
    Otherwise, log_10 is basically eliminated from the professional world.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: What should I assume when I see log?
  1. Now I see! (Replies: 16)

  2. Log of i (Replies: 11)