*e*) is usually referred to as

**ln**. The notation

**log**is often used to mean the logarithm to the base 10.

But, once you begin to study math beyond first year calculus, **log** is always used to mean the natural logarithm. The logarithm to the base 10 is virtually never used. If you should, for some reason, want to use a logarithm to the base 10, then you specify the base as a subscript to **log**; that is . So, for example, .

Note: If you need to review what a logarithm is and the basic properties of logs, click here.

This matters for three reasons. First, you need to know what I mean by **log** when I use the notation on this web site. When I use **log** I mean the natural logarith; that is **log** means . (The subscript on the **log** here may be a bit hard to read, but it is the number *e*; that is **log** to the base *e*.)

Second, you need to know what notation is used by statistics programs or other calculation programs (such as Microsoft Excel) you are using so that you can use the correct function or command name. *Most* statistics and mathematical calculation programs use **log** to mean . This is also the case for most computer programming languages such as C and C++.

But it is not true for what is probably the most widely used calculation program in the world: Excel. In Excel 2007, the natural logarithm is calculated using **ln( )**. If you use **log( )** you will get the logarithm to the base 10. The function **log10( )** also gives you the logarithm to the base 10.

Actually, in Excel 2007, the function **log( )** takes two arguments: the number you want logged and the base. But if you leave out the second argument, it defaults to base 10. Thus, in Excel 2007, **log( ) = log( ,10)**. If you wanted to use the Excel **log** function to calculate the natural log, you could do so with **log( ,exp(1))** since **exp(1)** gives the number *e*.

The key point of all of this is that you better know what the software you are using is actually calculating. To be certain, it is best to run some tests (see below).

The third reason that you need to understand what is meant by the **log** notation is that you often will need to “undo” the **log**. That is, you will need to compute the inverse function of the **log**.

Specifically, if **log** means logarithm to the base *e* then means that .

It is a very good idea to test the software you are using to make sure that you really know what **log** is doing. If **log** means the natural logarithm, then **log(exp(1))** will give the answer 1. If it gives something else, the **log** is not the natural logarithm. Note: in all computer languages (that I know of), **exp( )** means raise the number *e* to the power given by the argument. Thus, **exp(1)** = *e* = 2.718281828. You can, of course, check this as well.

In Excel 2007, **log(exp(1))** = 0.434294482 so you know that **log( )** is not giving you the natural logarithm.

### Help with Logarithms

If you do not know about logarithms or need a review that more or less starts from scratch, the series of videos at www.khanacademy.org are good. Start with the introductory video at the following link: Introduction to Logarithms. At the upper right-hand side of the embedded video screen, you will see a link for the next video. The next 9 videos (through “Solving Logarithmic Equations”) give a pretty complete introduction to logarithms.

Here is another good introduction (non-video): Introduction to Exponents and Logarithms.

**Questions or Comments?**

Any questions or comments? Please feel free to comment below. I am always wanting to improve this material.

I have had to implement a very simple “captcha” field because of spam comments, so be a bit careful about that. Enter your answer as a number (not a word). Also, save your comment before you submit it in case you make a mistake. You can use cntrl-A and then cntrl-C to save a copy on the clipboard. Particularly if your comment is long, I would hate for you to lose it.

http://en.wikipedia.org/wiki/Natural_logarithm

the link shows that Ln notation signifies Napier’s log, named after the eminent mathematician. I just thought I would contribute my two cents