661 is nontrivially palindromic in base 14.
661 is digitally balanced in base 2, because in such base it contains all the possibile digits an equal number of times.
It is the 11-th star number.
661 is an esthetic number in base 5, because in such base its adjacent digits differ by 1.
It is a weak prime.
It can be written as a sum of positive squares in only one way, i.e., 625 + 36 = 25^2 + 6^2 .It is also a bemirp because it and its reverse can be mirrored producing other two distinct primes, 199 and 991.
It is a cyclic number.
It is a magnanimous number.
661 is an undulating number in base 14.
It is a plaindrome in base 8 and base 13.
It is a nialpdrome in base 4, base 10 and base 11.
It is a zygodrome in base 4.
It is a congruent number.
It is a panconsummate number.
661 is the 12-th centered decagonal number.
It is an amenable number.
661 is an equidigital number, since it uses as much as digits as its factorization.
661 is an odious number, because the sum of its binary digits is odd.
The square root of 661 is about 25.7099202644. The cubic root of 661 is about 8.7109827387.
Subtracting from 661 its product of digits (36), we obtain a 4-th power (625 = 54).
The spelling of 661 in words is "six hundred sixty-one", and thus it is an aban number.