Fermat’s Last Theorem
$a^n+b^n=c^n$

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

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

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