21019991 has 8 divisors (see below), whose sum is σ = 21648000. Its totient is φ = 20398392.

The previous prime is 21019987. The next prime is 21019993. The reversal of 21019991 is 19991012.

It is a sphenic number, since it is the product of 3 distinct primes.

It is a cyclic number.

It is not a de Polignac number, because 21019991 - 2² = 21019987 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21019993) 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 7 ways as a sum of consecutive naturals, for example, 5510 + ... + 8508.

It is an arithmetic number, because the mean of its divisors is an integer number (2706000).

Almost surely, 2^21019991 is an apocalyptic number.

21019991 is a deficient number, since it is larger than the sum of its proper divisors (628009).

21019991 is a wasteful number, since it uses less digits than its factorization.

21019991 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 3205.

The product of its (nonzero) digits is 1458, while the sum is 32.

The square root of 21019991 is about 4584.7563730257. The cubic root of 21019991 is about 275.9799353544.

The spelling of 21019991 in words is "twenty-one million, nineteen thousand, nine hundred ninety-one".