666021 has 4 divisors (see below), whose sum is σ = 888032. Its totient is φ = 444012.

The previous prime is 666019. The next prime is 666023. The reversal of 666021 is 120666.

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 an interprime number because it is at equal distance from previous prime (666019) and next prime (666023).

It is not a de Polignac number, because 666021 - 2¹ = 666019 is a prime.

It is a D-number.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (666023) 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, 111001 + ... + 111006.

It is an arithmetic number, because the mean of its divisors is an integer number (222008).

2⁶⁶⁶⁰²¹ is an apocalyptic number.

It is an amenable number.

666021 is a deficient number, since it is larger than the sum of its proper divisors (222011).

666021 is a wasteful number, since it uses less digits than its factorization.

666021 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 222010.

The product of its (nonzero) digits is 432, while the sum is 21.

The square root of 666021 is about 816.1010966786. The cubic root of 666021 is about 87.3298352748.

Adding to 666021 its reverse (120666), we get a palindrome (786687).

It can be divided in two parts, 66 and 6021, that multiplied together give a triangular number (397386 = T₈₉₁).

The spelling of 666021 in words is "six hundred sixty-six thousand, twenty-one".