666666 is nontrivially palindromic in base 10 and base 16.
It is a nude number because it is divisible by every one of its digits.
666666 is a nontrivial repdigit in base 10.
It is a plaindrome in base 10.
It is a nialpdrome in base 7 and base 10.
It is a zygodrome in base 10.
It is a self number, because there is not a number n which added to its sum of digits gives 666666.
It is a congruent number.
It is a self-describing number.
2666666 is an apocalyptic number.
666666 is a gapful number since it is divisible by the number (66) formed by its first and last digit.
It is a practical number, because each smaller number is the sum of distinct divisors of 666666, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (995904).
It is a pseudoperfect number, because it is the sum of a subset of its proper divisors.
666666 is a wasteful number, since it uses less digits than its factorization.
666666 is an evil number, because the sum of its binary digits is even.
The square root of 666666 is about 816.4961726793. The cubic root of 666666 is about 87.3580173543.
The spelling of 666666 in words is "six hundred sixty-six thousand, six hundred sixty-six".