Amateurs are often interested in numbers with special
typographic qualities, such as the palindromes (which
read the same forward and backward). To define a vertical
and horizontal analog of the palindromes, we first recognise,
or pretend, that digits '0', '1', and '8' are each symmetric
both horizontally and vertically, and the digits '6' and '9'
are 180 degree rotations of each other. With these assumptions, a
tetradic (or 4-way) integer is a
reflectable palindromic strobogrammatic integer that is the same in
four ways; i.e., whether viewed from right to left, left
to right, top to bottom, or upside down. None of its
digits can be other than '0,' '1,' or '8.' The first
few tetradic primes are 11, 101, 181, 18181,
1008001, and 1180811.
(Obviously tetradic primes could be generalized to any
other radix, but even given the comment in the
entry on strobogrammatic primes, this would be going
See Also: Palindrome, Strobogrammatic, TriadicPrime
- H. Dubner and R. Ondrejka, "A PRIMEr on palindromes," J. Recreational Math., 26:4 (1994) 256--267.
- R. Ondrejka, "On tetradic or 4-way primes," J. Recreational Math., 21:1 (1989) 21-25.