A Tetradic Number is one which is both strobogrammatic and palindromic in nature.
It is the same when viewed from left to right, right to left, top to bottom or upside down.
This four-way symmetry explains the name, tetra- being the greek prefix for four.
The only digits that can be found in a tetradic number are 0, 1 and 8, since although matched
pairs of 6 and 9 can be used in strobogrammatic numbers, they won't yield a palindrome.
Thus the first few tetradic numbers are 0,1,8,11,88,101.
Given a tetradic number, a larger one can always be generated by adding
another tetradic number to each end, retaining the symmetry.
There are tetradic primes, the first half dozen being 11, 101, 181, 18181, 1008001, and 1180811.
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