1666821 has 16 divisors (see below), whose sum is σ = 2428160. Its totient is φ = 1010880.

The previous prime is 1666811. The next prime is 1666823. The reversal of 1666821 is 1286661.

It is not a de Polignac number, because 1666821 - 2⁵ = 1666789 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (1666823) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 2811 + ... + 3351.

It is an arithmetic number, because the mean of its divisors is an integer number (151760).

2^1666821 is an apocalyptic number.

It is an amenable number.

1666821 is a deficient number, since it is larger than the sum of its proper divisors (761339).

1666821 is a wasteful number, since it uses less digits than its factorization.

1666821 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 636.

The product of its digits is 3456, while the sum is 30.

The square root of 1666821 is about 1291.0542203951. The cubic root of 1666821 is about 118.5667696847.

The spelling of 1666821 in words is "one million, six hundred sixty-six thousand, eight hundred twenty-one".