21019891 has 4 divisors (see below), whose sum is σ = 21697984. Its totient is φ = 20341800.

The previous prime is 21019879. The next prime is 21019903. The reversal of 21019891 is 19891012.

It is a semiprime because it is the product of two primes.

It is an interprime number because it is at equal distance from previous prime (21019879) and next prime (21019903).

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21019891 is a prime.

It is a Harshad number since it is a multiple of its sum of digits (31), and also a Moran number because the ratio is a prime number: 678061 = 21019891 / (2 + 1 + 0 + 1 + 9 + 8 + 9 + 1).

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21019841) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 339000 + ... + 339061.

It is an arithmetic number, because the mean of its divisors is an integer number (5424496).

Almost surely, 2^21019891 is an apocalyptic number.

21019891 is a deficient number, since it is larger than the sum of its proper divisors (678093).

21019891 is an equidigital number, since it uses as much as digits as its factorization.

21019891 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 678092.

The product of its (nonzero) digits is 1296, while the sum is 31.

The square root of 21019891 is about 4584.7454673079. The cubic root of 21019891 is about 275.9794977069.

The spelling of 21019891 in words is "twenty-one million, nineteen thousand, eight hundred ninety-one".