20561921 has 4 divisors (see below), whose sum is σ = 20573952. Its totient is φ = 20549892.

The previous prime is 20561917. The next prime is 20561923. The reversal of 20561921 is 12916502.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4, and also a brilliant number, because the two primes have the same length.

It is a cyclic number.

It is not a de Polignac number, because 20561921 - 2² = 20561917 is a prime.

It is a Duffinian number.

It is a Curzon number.

It is not an unprimeable number, because it can be changed into a prime (20561923) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 2921 + ... + 7046.

It is an arithmetic number, because the mean of its divisors is an integer number (5143488).

It is a Proth number, since it is equal to 1255 ⋅ 2¹⁴ + 1 and 1255 < 2¹⁴.

Almost surely, 2^20561921 is an apocalyptic number.

It is an amenable number.

20561921 is a deficient number, since it is larger than the sum of its proper divisors (12031).

20561921 is an equidigital number, since it uses as much as digits as its factorization.

20561921 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 12030.

The product of its (nonzero) digits is 1080, while the sum is 26.

The square root of 20561921 is about 4534.5254437482. The cubic root of 20561921 is about 273.9604654630.

It can be divided in two parts, 205 and 61921, that added together give a palindrome (62126).

The spelling of 20561921 in words is "twenty million, five hundred sixty-one thousand, nine hundred twenty-one".