21787561 has 8 divisors (see below), whose sum is σ = 22500864. Its totient is φ = 21080880.

The previous prime is 21787543. The next prime is 21787567. The reversal of 21787561 is 16578712.

It is a sphenic number, since it is the product of 3 distinct primes.

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21787561 is a prime.

It is a Harshad number since it is a multiple of its sum of digits (37).

It is a Duffinian number.

It is a self number, because there is not a number n which added to its sum of digits gives 21787561.

It is not an unprimeable number, because it can be changed into a prime (21787567) 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 7 ways as a sum of consecutive naturals, for example, 5526 + ... + 8608.

It is an arithmetic number, because the mean of its divisors is an integer number (2812608).

Almost surely, 2^21787561 is an apocalyptic number.

It is an amenable number.

21787561 is a deficient number, since it is larger than the sum of its proper divisors (713303).

21787561 is a wasteful number, since it uses less digits than its factorization.

21787561 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 3311.

The product of its digits is 23520, while the sum is 37.

The square root of 21787561 is about 4667.7147513532. The cubic root of 21787561 is about 279.2991015607.

It can be divided in two parts, 217 and 87561, that added together give a palindrome (87778).

The spelling of 21787561 in words is "twenty-one million, seven hundred eighty-seven thousand, five hundred sixty-one".