20181561 has 4 divisors (see below), whose sum is σ = 26908752. Its totient is φ = 13454372.

The previous prime is 20181559. The next prime is 20181563. The reversal of 20181561 is 16518102.

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 an interprime number because it is at equal distance from previous prime (20181559) and next prime (20181563).

It is a cyclic number.

It is not a de Polignac number, because 20181561 - 2¹ = 20181559 is a prime.

It is a Curzon number.

It is not an unprimeable number, because it can be changed into a prime (20181563) 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, 3363591 + ... + 3363596.

It is an arithmetic number, because the mean of its divisors is an integer number (6727188).

Almost surely, 2^20181561 is an apocalyptic number.

It is an amenable number.

20181561 is a deficient number, since it is larger than the sum of its proper divisors (6727191).

20181561 is an equidigital number, since it uses as much as digits as its factorization.

20181561 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 6727190.

The product of its (nonzero) digits is 480, while the sum is 24.

The square root of 20181561 is about 4492.3892306878. The cubic root of 20181561 is about 272.2606758827.

Adding to 20181561 its reverse (16518102), we get a palindrome (36699663).

The spelling of 20181561 in words is "twenty million, one hundred eighty-one thousand, five hundred sixty-one".