A number n such that the "LED representation" of n (i.e., the arrangement of horizonal and vertical lines seen on a digital clock or pocket calculator), n upside down, n in a mirror, and n upside-down-and-in-a-mirror are all primes. The digits of n are therefore restricted to 0, 1, 2, 5, and 8. The first few dihedral primes are 2, 5, 11, 101, 181, 1181, 1811, 18181, 108881, 110881, 118081, 120121, ... (OEIS A134996).

tetradic number