  We study the equivalence between the Ring Learning With Errors and Polynomial
Learning With Errors problems for cyclotomic number fields,namely: we prove
that both problems are equivalent via a polynomial noise increase as long as
the number of distinct primes dividing the conductor is kept constant. We
refine our bound in the case where the conductor is divisible by at most three
primes and we give an asymptotic subexponential formula for the condition
number of the attached Vandermonde matrix valid for arbitrary degree.
