For the lattice below. What is the Greatest Lower Bound of $a_2$ and $a_4$ (i.e., $a_2 \land a_4$) and $a_3$ and $a_4$ (i.e. $a_3 \land a_4$)?
I thought it was $a_2 \land a_4 = a_2$ and $a_3 \land a_4= a_3$, but I got it wrong on the homework and I don't know what the right answer is.
Can someone tell me what the Greatest Lower Bound is and why?