Formulating constraints to an integer programming problem

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.

HINT

1. Learn how to define $x = b \text{ or } c$ .
2. Similarly, define $y = x \text{ or } d$.
3. Use the technique you mentioned you know to force $a$ if $y$.

UPDATE

What does $x \ge b$ enforce in the relationship between $x$ and $b$? What about $x \ge b$ and $x \ge c$ simultaneously?

• Here is my take at this, please let me know if my logic is correct: $x \leq b+c$ and $y \leq x+d$ and $a=y$ where all the variables are binary – mhlzzz May 18 '17 at 20:43
• @mhlzzz if you just want $b \text{ or } c \text{ or } d = 1 \implies a = 1$ then it's much simpler – gt6989b May 18 '17 at 20:46
• @mhlzzz your solution does not force anything, in fact, if both $b,c$ are true, $x$ can be anything. See another update – gt6989b May 18 '17 at 20:48
• Regarding your update, what I gather is that $x=1$ if $b=1$ or $c=1$ but if $b=0$ and $c=0$ then $x$ can be $0$ or $1$ – mhlzzz May 18 '17 at 20:52
• @mhlzzz yes, i figured from your own answer you figured ot where i was leading you. – gt6989b May 19 '17 at 17:06

I did $a\geq b$,$a\geq c$,$a\geq d$. To show if any of $b$, $c$, or $d$ are $1$ (true) then $a$ must be as well.