20156561 has 4 divisors (see below), whose sum is σ = 20585472. Its totient is φ = 19727652.

The previous prime is 20156557. The next prime is 20156567. The reversal of 20156561 is 16565102.

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.

It is a cyclic number.

It is not a de Polignac number, because 20156561 - 2² = 20156557 is a prime.

It is a Duffinian number.

It is a Curzon number.

It is a self number, because there is not a number n which added to its sum of digits gives 20156561.

It is not an unprimeable number, because it can be changed into a prime (20156567) 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, 214385 + ... + 214478.

It is an arithmetic number, because the mean of its divisors is an integer number (5146368).

Almost surely, 2^20156561 is an apocalyptic number.

It is an amenable number.

20156561 is a deficient number, since it is larger than the sum of its proper divisors (428911).

20156561 is an equidigital number, since it uses as much as digits as its factorization.

20156561 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 428910.

The product of its (nonzero) digits is 1800, while the sum is 26.

The square root of 20156561 is about 4489.6058847075. The cubic root of 20156561 is about 272.1482080486.

The spelling of 20156561 in words is "twenty million, one hundred fifty-six thousand, five hundred sixty-one".