A tetradic (or four-way) number is a nonnegative integer that is a palindromic number in
base 10 and has only 0, 1, and 8 as digits.
The glyphs 0, 1, and 8 are assumed to be symmetric about both a horizontal and a
vertical axis, with 1 written as a single stroke and 8 as a symmetric pair of loops.
Under this convention, the numeral remains unchanged under horizontal or vertical
reflection or a 
rotation. The first
few tetradic numbers are 0, 1, 8, 11, 88, 101, 111, 181, 808, 818, ... (OEIS A006072).
The first few tetradic primes are 11, 101, 181, 18181, 1008001, 1180811, 1880881, 1881881, ... (OEIS A068188). The largest known tetradic prime number as of Apr. 2010 was
 |
where 
is a repunit. This number has 180055 decimal
digits (PrimePages 2009).
." Sep. 8, 2009.