DISPLAYED EQUATIONS: NUMBERING AND NAMING THEM

We have now discussed the macros available for defining the basic parts of a paper and appropriately titling them. Now let us turn to the most important stuff – other than text – which appears in each section; namely equations. If you've done your homework and read Knuth or First Grade TEX you know that equations are typed in math mode and that you let TEX know that you are in math mode by typing either $ or $$ . Enclosing an equation inside a pair of single $ signs puts it in ordinary mode; enclosing the same equation between matching pairs of $$ signs puts it in display mode. Regular math mode is used for inserting equations into a line of text. Since you have done your homework we don't have to say anything about this, since PHYZZX doesn't play with ordinary math mode at all. Display mode is used for generating equations which are to stand out from the ordinary text. It allows you to type $$ (x+y)(x-y) = xˆ 2 - yˆ 2 $$ in order to get

(x + y)(x - y) = x² - y²

This is called a displayed equation.

Displayed equations are used in all journals and are usually numbered. In PLAIN TEX there is a built in capability for allowing you to generate equation numbers; it is the macro eqno. If you type $$ (x+y)(x-y) = xˆ 2 - yˆ 2 eqno(13) $$ you get

(x + y)(x - y) = x² - y² $\displaystyle \eqno$ (13)

This example shows what the command eqno does. It makes the material appearing to the right of this command into a label for the displayed equation and sticks it in the right hand side of the line. There is also the PLAIN TEX command leqno which does the same thing but puts the equation number on the left. For example, typing $$ 7x+12 = 24 leqno (13) $$ produces

7x + 12 = 24 $\displaystyle \leqno$ (13)

So, you ask, what does PHYZZX have to do with this? Well the answer to that question depends upon who you are. If you are typing someone else's handwritten manuscript which is in final proofread form, then these macros for putting in equation numbers are all you need. If, however, you are the author of the paper, typing it in yourself, either from a rough copy or composing it directly at the terminal, then these two commands leave a great deal to be desired.

To begin with, if you are anything like me, when composing at the typewriter you usually forget the number of the last equation you have typed. Of course you can always go back and look it up, but this is time consuming and sometimes difficult to do. In addition if, as is usually the case, you expect the manuscript to be changed after it is typed, then it is highly likely that equations will be added, deleted and moved around. In that case, if you have used eqno or leqno you will have to go through the text and change all of the equation numbers and all references to those numbers each time there is a revision. Clearly this can get to be a big nuisance. Wouldn't it be nice if TEX would just take care of numbering all of the equations properly each time it prints the manuscript taking into account all of the revisions automatically.

Joy of joys it can be done!

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