20171561 has 4 divisors (see below), whose sum is σ = 20188500. Its totient is φ = 20154624.

The previous prime is 20171551. The next prime is 20171603. The reversal of 20171561 is 16517102.

It is a semiprime because it is the product of two primes.

It can be written as a sum of positive squares in 2 ways, for example, as 9803161 + 10368400 = 3131^2 + 3220^2 .

It is a cyclic number.

It is not a de Polignac number, because 20171561 - 2⁶ = 20171497 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (20171531) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 6536 + ... + 9113.

It is an arithmetic number, because the mean of its divisors is an integer number (5047125).

Almost surely, 2^20171561 is an apocalyptic number.

It is an amenable number.

20171561 is a deficient number, since it is larger than the sum of its proper divisors (16939).

20171561 is a wasteful number, since it uses less digits than its factorization.

20171561 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 16938.

The product of its (nonzero) digits is 420, while the sum is 23.

The square root of 20171561 is about 4491.2760992840. The cubic root of 20171561 is about 272.2156999008.

Adding to 20171561 its reverse (16517102), we get a palindrome (36688663).

The spelling of 20171561 in words is "twenty million, one hundred seventy-one thousand, five hundred sixty-one".