666661 has 4 divisors (see below), whose sum is σ = 668800. Its totient is φ = 664524.

The previous prime is 666649. The next prime is 666667. The reversal of 666661 is 166666.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4.

It is a cyclic number.

It is not a de Polignac number, because 666661 - 2¹¹ = 664613 is a prime.

It is a Duffinian number.

It is a nialpdrome in base 7 and base 10.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (666667) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 501 + ... + 1258.

It is an arithmetic number, because the mean of its divisors is an integer number (167200).

2⁶⁶⁶⁶⁶¹ is an apocalyptic number.

It is an amenable number.

666661 is a deficient number, since it is larger than the sum of its proper divisors (2139).

666661 is a wasteful number, since it uses less digits than its factorization.

666661 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 2138.

The product of its digits is 7776, while the sum is 31.

The square root of 666661 is about 816.4931108099. The cubic root of 666661 is about 87.3577989585.

The spelling of 666661 in words is "six hundred sixty-six thousand, six hundred sixty-one".