1666643721 has 16 divisors (see below), whose sum is σ = 2248436736. Its totient is φ = 1098018576.

The previous prime is 1666643689. The next prime is 1666643743. The reversal of 1666643721 is 1273466661.

It is not a de Polignac number, because 1666643721 - 2⁵ = 1666643689 is a prime.

It is not an unprimeable number, because it can be changed into a prime (1666443721) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (17) of ones.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 150066 + ... + 160788.

It is an arithmetic number, because the mean of its divisors is an integer number (140527296).

Almost surely, 2^1666643721 is an apocalyptic number.

It is an amenable number.

1666643721 is a deficient number, since it is larger than the sum of its proper divisors (581793015).

1666643721 is a wasteful number, since it uses less digits than its factorization.

1666643721 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 11332.

The product of its digits is 217728, while the sum is 42.

The square root of 1666643721 is about 40824.5480195433. The cubic root of 1666643721 is about 1185.6256604525.

The spelling of 1666643721 in words is "one billion, six hundred sixty-six million, six hundred forty-three thousand, seven hundred twenty-one".