a) Show that if p and q are distinct odd primes, then pq is a pseudoprime to the base 2 if and only if ord_{q}2 \mid (p-1) and ord_{p}2 \mid (q-1)

b) Use part (a) to decide which of the following integers are pseudoprimes to the base 2: 13*67, 19*73, 23*89, 29*97.