I would like to know how many mathematical constant are there? I saw this link and I know the names.
- Who can define a mathematical constant?
- Someone can say that $\int_{x=0}^{1}e^{-\cos(x)}dx\sim.435351$ is called $\hat o$ and it is a new mathematical constant?
- Are there mathematical constant that are not useful?
- And what are the most important mathematical constants?
$\pi$, $e$, $\gamma$ (Euler's Constant) stand out as some of the more important mathematical constants. If we wanted we could declare any real number as a ``mathematical constant". However, the numbers that bear that name have been found useful in some context, and most commonly they have been found useful in many contexts.
So the answer to your second question is: there are not any "mathematical constants" that are not useful.
The answer to your first question is that the community defines mathematical constants. Generally people who work in a field notice that a number is useful and is appearing frequently and that is how a number comes about as a "mathematical constant".