Show that if n is an Euler pseudoprime to the bases a and b, then n is an Euler pseudoprime to the base ab.

If A = +/- 1 modn and B = +/-1 modn then
multiplying conguences gives AB = +/- 1 modn
We let A = a^((n-1)/2) and B = b^((n-1)/2)
then AB = (ab)^((n-1)/2) so n is an Euler pseudoprime
to the base ab as required.