The symbols ≪ and ≫ may be confusing the first time you see them, but they’re very handy.
The symbol ≪ means “much less than, and its counterpart ≫ means “much greater than”. Here’s a little table showing how to produce the symbols.
|-------------------+---------+-------+------|
| | Unicode | LaTeX | HTML |
|-------------------+---------+-------+------|
| Much less than | U+226A | \ll | ≪ |
| Much greater than | U+226B | \gg | ≫ |
|-------------------+---------+-------+------|
Of course “much” depends on context. Is 5 much less than 7? It is if you’re describing the height of people in feet, but maybe not in the context of prices of hamburgers in dollars.
Sometimes you’ll see ≫, or more likely >> (two greater than symbols), as slang for “is much better than.” For example, someone might say “prototype >> powerpoint” to convey that a working prototype is much better than a PowerPoint pitch deck.
The symbols ≪ and ≫ can make people uncomfortable because they’re insider jargon. You have to know the context to understand how to interpret them, but they’re very handy if you are an insider. All jargon is like this.
Below are some examples of ≪ and ≫ in practice.
Square root approximation
You might see somewhere that for |b| ≪ a, the following approximation holds:
So when is |b| much less than a? That’s up to you. If, in your context, you decide that b/a is small, the approximation error will be an order of magnitude smaller.
Suppose you need to know √103 to a couple decimal places. Here a = 100 and b = 3. The ratio b/a = 0.03, and your error should be small relative to 0.03, so the approximation above should be good enough. Let’s see if that’s right.
The approximation above gives
√103 ≈ √100 + 3/2√100 = 10 + 3/20 = 10.15
and the exact value of √103 is 10.14889…, and so we did get two correct decimal places, and we nearly got three.
Sine approximation
Rather than saying a variable is “small,” we might say it is much less than 1. For example, you may see
sin θ ≈ θ
for |θ| ≪ 1. If θ is small, the error in the approximation above is very small.
A few years I wrote a 700-word blog post unpacking in detail what the previous sentence means. A lot of people memorize “You can replace sin θ with θ for small angles” without thoroughly understanding what this means. How small is small enough? The post explains how to know.
Stirling’s formula
Sometimes you see something like n ≫ 1 to indicate that n must be large. For example, Stirling’s formula for factorials says
for n ≫ 1. For instance, if n = 10, the approximation above has an error of less than 1%.
Note that the approximation error above is small relative to the exact value. The relative error is small, not the absolute error. The error in the example is more than 30,000, but this value is small relative to 10! = 3,628,800.
Asymmetry between small and large
It’s often harder to tell from context when something is large than when it is small.
If an approximation holds for |x| ≪ 1, there’s often an implicit power series in the background, and the error is on the order of x². That’s the case in our square root approximation above. The sine approximation is even better, with error on the order of x³.
But if an approximation holds for x ≫ 1, there’s often an implicit asymptotic series in the background, and these are more subtle. You likely need more context to how large x needs to be for a particular application.