20721561 has 24 divisors (see below), whose sum is σ = 32596704. Its totient is φ = 11672640.

The previous prime is 20721553. The next prime is 20721583. The reversal of 20721561 is 16512702.

It is not a de Polignac number, because 20721561 - 2³ = 20721553 is a prime.

It is not an unprimeable number, because it can be changed into a prime (20721511) by changing a digit.

It is a polite number, since it can be written in 23 ways as a sum of consecutive naturals, for example, 9766 + ... + 11696.

It is an arithmetic number, because the mean of its divisors is an integer number (1358196).

Almost surely, 2^20721561 is an apocalyptic number.

20721561 is a gapful number since it is divisible by the number (21) formed by its first and last digit.

It is an amenable number.

20721561 is a deficient number, since it is larger than the sum of its proper divisors (11875143).

20721561 is a wasteful number, since it uses less digits than its factorization.

20721561 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 2021 (or 2014 counting only the distinct ones).

The product of its (nonzero) digits is 840, while the sum is 24.

The square root of 20721561 is about 4552.0941334731. The cubic root of 20721561 is about 274.6676359333.

The spelling of 20721561 in words is "twenty million, seven hundred twenty-one thousand, five hundred sixty-one".