20991021 has 8 divisors (see below), whose sum is σ = 28516320. Its totient is φ = 13729872.

The previous prime is 20991017. The next prime is 20991023. The reversal of 20991021 is 12019902.

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

It is not a de Polignac number, because 20991021 - 2² = 20991017 is a prime.

It is a Curzon number.

It is a junction number, because it is equal to n+sod(n) for n = 20990982 and 20991000.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (20991023) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 65851 + ... + 66168.

It is an arithmetic number, because the mean of its divisors is an integer number (3564540).

Almost surely, 2^20991021 is an apocalyptic number.

It is an amenable number.

20991021 is a deficient number, since it is larger than the sum of its proper divisors (7525299).

20991021 is a wasteful number, since it uses less digits than its factorization.

20991021 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 132075.

The product of its (nonzero) digits is 324, while the sum is 24.

The square root of 20991021 is about 4581.5959009935. The cubic root of 20991021 is about 275.8530907942.

The spelling of 20991021 in words is "twenty million, nine hundred ninety-one thousand, twenty-one".