Important numbers for differential equations:
ⓐ Nontrivial solutions of ẏ = y are exponential functions with base e.
ⓑ Nontrivial solutions of ÿ = -y are periodic functions with period 𝜏.
If you have a fediverse account, you can quote this note from your own instance. Search https://mastodon.social/users/hcschuetz/statuses/115865620877159538 on your instance and quote it. (Note that quoting is not supported in Mastodon.)