In relation to the thread Proving that the conversion from CNF to DNF is NP-Hard (and a related Math thread):
How about the other direction, from DNF to CNF? Is it easy or hard?
On Page 2 of this paper, they seem to hint that both directions are equally hard when they say "We are interested in the maximal blow-up of size when switching from the CNF representation to the DNF representation (or vice versa)".
But DNF-SAT is in P and CNF-SAT is NP-complete. So given a DNF expression $\phi_1$, there should be an equisatisfiable CNF expression $\phi_2$ whose length is polynomial in the length of $\phi_1$. And the $\phi_1 \to \phi_2$ conversion can be done in poly time. Is this correct?
Edit: Changed equivalent to equisatisfiable (that is, additional variables are allowed in $\phi_2$).