Fermat’s Last Theorem
$a^n+b^n=c^n$

Bellman Equation for Dynamic Programming:
$J^{\ast }(i)=g(i,u)+\sum _{j=0}^n{p}_{ij}(u)J^{\ast }(j)$

Of course, the famous mass-energy equation from Einstein:
$E=m{c}^2$

I will keep updating this post ad-$\mathrm{\infty }$ …