21631561 has 4 divisors (see below), whose sum is σ = 24721792. Its totient is φ = 18541332.

The previous prime is 21631541. The next prime is 21631573. The reversal of 21631561 is 16513612.

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 21631561 - 2¹¹ = 21629513 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21631501) 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, 1545105 + ... + 1545118.

It is an arithmetic number, because the mean of its divisors is an integer number (6180448).

Almost surely, 2^21631561 is an apocalyptic number.

It is an amenable number.

21631561 is a deficient number, since it is larger than the sum of its proper divisors (3090231).

21631561 is an equidigital number, since it uses as much as digits as its factorization.

21631561 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 3090230.

The product of its digits is 1080, while the sum is 25.

The square root of 21631561 is about 4650.9741990254. The cubic root of 21631561 is about 278.6309058438.

The spelling of 21631561 in words is "twenty-one million, six hundred thirty-one thousand, five hundred sixty-one".