[Cfp-interest 3359] Euler's Number - e a.k.a. M_E or M_El in C

[Damian McGuckin]

or diverging somewhat, in C++, e_v if one includes C++20's <numbers>.

I am curious how often this constant is ever used in people's programs for 
anything other than testing purposens?

I have the same question about M_2_SQRTPI outside of its use with source
code for implementations of the normal dstribution functon.

Just curious.

Thanks - Damian