16661721021 has 16 divisors (see below), whose sum is σ = 24266062848. Its totient is φ = 10085166000.

The previous prime is 16661721007. The next prime is 16661721023. The reversal of 16661721021 is 12012716661.

It is not a de Polignac number, because 16661721021 - 2²⁶ = 16594612157 is a prime.

It is a super-2 number, since 2×16661721021² (a number of 21 digits) contains 22 as substring.

It is a Harshad number since it is a multiple of its sum of digits (33).

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (16661721023) by changing a digit.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 294805 + ... + 346746.

It is an arithmetic number, because the mean of its divisors is an integer number (1516628928).

Almost surely, 2^16661721021 is an apocalyptic number.

16661721021 is a gapful number since it is divisible by the number (11) formed by its first and last digit.

It is an amenable number.

16661721021 is a deficient number, since it is larger than the sum of its proper divisors (7604341827).

16661721021 is a wasteful number, since it uses less digits than its factorization.

16661721021 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 642352.

The product of its (nonzero) digits is 6048, while the sum is 33.

The spelling of 16661721021 in words is "sixteen billion, six hundred sixty-one million, seven hundred twenty-one thousand, twenty-one".