I am trying to model the following constraint using binary variables $a$, $b$, $c$ and $d$:
$a$ will be true if at least one of $b$,$c$, or $d$ is true.
I know how to do $a$ if $b$ but I am having a very hard time introducing the "at least one of" part.
What I tried but realized didn't work is $a \leq b+c+d$ because this does mean $a$ can only be true if one of he others is true but it does not mean $a$ will be true if one of the others is true.
UPDATE: I just solved it! I did $a\geq b$,$a\geq c$,$a\geq d$. To show if any are $1$ (true) then $a$ must be as well.