Equations and numbers

Formatting numbers
"One hundred and forty-two thousand, six hundred and twenty-one" is not something that you would expect to find in a technical paper, or indeed anywhere other than a bank draft (you should be so lucky). However, because it's right to write 142,621 (note the comma) doesn't mean that it's right to write "the system consists of 1 processor, 1 memory system and 2 storage devices". You'd expect to see "... one processor, one memory system and two storage devices".
So, somewhere between one and 142,621, we need to change between words and figures. You can choose the cut-off point for yourself, but 10 is a common value: so you write words for one to nine, and figures for 10 up. Sometimes, you might even be justified in writing "nought", but nought and one are special cases which we will deal with in a moment.
*Naturally, there are exceptions, such as the parts of the paper itself. No one is suggesting you stop writing "Fig. 4", and write "Figure Four" instead.
*Another exception is for numbers in a technical context. No one is going to write 2.5, or any other real number, in words, just because it happens to be between 1 and 10. Note that I just wrote "1". Even integers should be written as figures if they are effectively constants in some mathematical construct (here, an implied inequality), even though counting the constructs themselves would be subject to the original convention about figures and words. So, you might write "there are two terms, and they sum to 2.0" If you want to emphasize that you're talking about real numbers, it is usually better to add a decimal, even if it's a zero.
*Nought and one are special cases, not least because "0" and "1" look alarmingly like a capital "O" and lower-case "l". A lot of "0"s and "1"s in text doesn't make it easy to read. One solution is to use "zero" (as I just did a few lines ago) and "unity", which both have honourable mathematical careers behind them.
However, it is often possible to eliminate the need to write about "one" at all. The corrected version of the example above is more plausibly written "the system consists of a processor, memory system and two storage devices."
Similarly with zero. A phrase like "there are 0 bytes of code in this routine" is easily rewritten as "this routine contains no code". Words like "empty", "null", "void" and "absent" can all be deployed in substituting for zero.

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