21632561 has 4 divisors (see below), whose sum is σ = 21729792. Its totient is φ = 21535332.

The previous prime is 21632557. The next prime is 21632563. The reversal of 21632561 is 16523612.

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 21632561 - 2² = 21632557 is a prime.

It is an alternating number because its digits alternate between even and odd.

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 21632561.

It is not an unprimeable number, because it can be changed into a prime (21632563) 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, 48281 + ... + 48726.

It is an arithmetic number, because the mean of its divisors is an integer number (5432448).

Almost surely, 2^21632561 is an apocalyptic number.

It is an amenable number.

21632561 is a deficient number, since it is larger than the sum of its proper divisors (97231).

21632561 is an equidigital number, since it uses as much as digits as its factorization.

21632561 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 97230.

The product of its digits is 2160, while the sum is 26.

The square root of 21632561 is about 4651.0817021420. The cubic root of 21632561 is about 278.6351993637.

The spelling of 21632561 in words is "twenty-one million, six hundred thirty-two thousand, five hundred sixty-one".