16721991 has 64 divisors (see below), whose sum is σ = 29998080. Its totient is φ = 9072000.

The previous prime is 16721981. The next prime is 16721993. The reversal of 16721991 is 19912761.

It is not a de Polignac number, because 16721991 - 2⁹ = 16721479 is a prime.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (16721993) by changing a digit.

It is a polite number, since it can be written in 63 ways as a sum of consecutive naturals, for example, 235486 + ... + 235556.

It is an arithmetic number, because the mean of its divisors is an integer number (468720).

Almost surely, 2^16721991 is an apocalyptic number.

16721991 is a gapful number since it is divisible by the number (11) formed by its first and last digit.

16721991 is a deficient number, since it is larger than the sum of its proper divisors (13276089).

16721991 is a wasteful number, since it uses less digits than its factorization.

16721991 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 165 (or 159 counting only the distinct ones).

The product of its digits is 6804, while the sum is 36.

The square root of 16721991 is about 4089.2531102880. The cubic root of 16721991 is about 255.7188023623.

It can be divided in two parts, 1672 and 1991, that added together give a palindrome (3663).

The spelling of 16721991 in words is "sixteen million, seven hundred twenty-one thousand, nine hundred ninety-one".