1666721 has 4 divisors (see below), whose sum is σ = 1904832. Its totient is φ = 1428612.

The previous prime is 1666711. The next prime is 1666727. The reversal of 1666721 is 1276661.

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 1666721 - 2⁶ = 1666657 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (1666727) 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 3 ways as a sum of consecutive naturals, for example, 119045 + ... + 119058.

It is an arithmetic number, because the mean of its divisors is an integer number (476208).

2^1666721 is an apocalyptic number.

It is an amenable number.

1666721 is a deficient number, since it is larger than the sum of its proper divisors (238111).

1666721 is an equidigital number, since it uses as much as digits as its factorization.

1666721 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 238110.

The product of its digits is 3024, while the sum is 29.

The square root of 1666721 is about 1291.0154917738. The cubic root of 1666721 is about 118.5643985215.

The spelling of 1666721 in words is "one million, six hundred sixty-six thousand, seven hundred twenty-one".