It is sometimes restricted to non-trivial undulating numbers, which are required to have at least three digits and A ≠ B.
The first few undulating numbers are: For the full sequence of undulating numbers, see OEIS: A033619.
Some larger undulating numbers are: 1010, 80808, 171717, 989898989.
In every base, all undulating primes having at least three digits have an odd number of digits and are palindromic primes.
The undulating primes in base 10 are: This number theory-related article is a stub.