The relations ~ and ≍ are frequently used in math and computer science, at least within number theory and analysis of algorithms. What is their origin?


Suppose $g(x)$ is an eventually-positive function of $x$. Then

$$ f(x) \sim g(x) $$

if and only if

$$ \lim_{x\to\infty} f(x)/g(x) = k $$ with $0<k<\infty,$ while

$$ f(x) \asymp g(x) $$ if and only if $$ 0 < \liminf_{x\to\infty} f(x)/g(x) \le \limsup_{x\to\infty} f(x)/g(x) < \infty. $$

Note that the defintion of ~ replaces the $\le$ for $=$ in the above.

Related work

I believe the related notations $\ll$ and $\gg$ are due to Vinogradov, while $O/o/\Omega/\omega$ are due to Hardy and Littlewood. (I've skimmed through Hardy's Orders of Infinity which suggests that he and Paul Du Bois-Reymond did some early work here but I don't think that either of these notations originated there.)

I was not able to find these in either of these online resources:


1 Answer 1


The notation $\sim$ does not mean that ratio $f(x)/g(x)$ has some positive limit $k$, but that it has limit $1$, e.g., $2x + \sqrt{x} \sim 2x$. What math books do you know that define $\sim$ in the way you did? It would allow us to write $2x+\sqrt{x} \sim x$, which is a meaning of $\sim$ that I've never seen.

This notation $\sim$ is due to Landau: see page 62 of the first volume of his Handbuch der Lehre von der Verteilung der Primzahlen (1909). It's available at archive.org here.

The $O$-notation and $o$-notation are not due to Hardy and Littlewood:

  1. The $o$-notation is due to Landau in the same book I already mentioned, on page 61.

  2. The $O$-notation is due to Bachmann on p. 401 of his book Die Analytische Zahlentheorie (1894) here.

  • $\begingroup$ Probably $\Omega$ and $\omega$ are more recent than Hardy and Littlewood. $\endgroup$ Jul 16, 2023 at 1:13

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