Atetradicnumber, also known as afour-waynumber, is a number that remains the same when flipped back to front, flipped front to back, mirrored up-down, or flipped up-down. The only numbers that remain the same which turned up-side-down or mirrored are 0, 1, and 8, so a tetradic number is apalindromic number containing only 0, 1, and 8 as digits. (This is dependent on the use of a handwriting style or font in which these digitsare symmetrical, as well on the use ofArabic numerals in the first place.) The first few tetradic numbers are 1, 8, 11, 88, 101, 111, 181, 808, 818, ... (OEIS A006072).[1][2][3][4]
Tetradic numbers are also known as four-way numbers because they have four-waysymmetry and can flipped back to front, flipped front to back, mirrored up-down, or flipped up-down and always stay the same. The four-way symmetry explains the name, due totetra- being the Greek prefix for four. Tetradic numbers are bothstrobogrammatic andpalindromic.[3][4]
A larger tetradic number can always be generated by adding another tetradic number to each end, retaining the symmetry.
Tetradic primes are a specific type of tetradic number defined as tetradic numbers that are alsoprime numbers. The first few tetradic primes are 11, 101, 181, 18181, 1008001, 1180811, 1880881, 1881881, ... (OEIS A068188).[5][6][7][8][9][10]