1665721 has 4 divisors (see below), whose sum is σ = 1668352. Its totient is φ = 1663092.

The previous prime is 1665709. The next prime is 1665757. The reversal of 1665721 is 1275661.

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, and also a brilliant number, because the two primes have the same length.

It is a cyclic number.

It is not a de Polignac number, because 1665721 - 2⁵ = 1665689 is a prime.

It is a Duffinian number.

It is a self number, because there is not a number n which added to its sum of digits gives 1665721.

It is not an unprimeable number, because it can be changed into a prime (1665701) 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, 280 + ... + 1846.

It is an arithmetic number, because the mean of its divisors is an integer number (417088).

2^1665721 is an apocalyptic number.

It is an amenable number.

1665721 is a deficient number, since it is larger than the sum of its proper divisors (2631).

1665721 is a wasteful number, since it uses less digits than its factorization.

1665721 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 2630.

The product of its digits is 2520, while the sum is 28.

The square root of 1665721 is about 1290.6281416427. The cubic root of 1665721 is about 118.5406816709.

The spelling of 1665721 in words is "one million, six hundred sixty-five thousand, seven hundred twenty-one".