21289561 has 4 divisors (see below), whose sum is σ = 21316624. Its totient is φ = 21262500.

The previous prime is 21289547. The next prime is 21289573. The reversal of 21289561 is 16598212.

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 21289561 - 2¹³ = 21281369 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21289511) 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, 12315 + ... + 13936.

It is an arithmetic number, because the mean of its divisors is an integer number (5329156).

Almost surely, 2^21289561 is an apocalyptic number.

It is an amenable number.

21289561 is a deficient number, since it is larger than the sum of its proper divisors (27063).

21289561 is an equidigital number, since it uses as much as digits as its factorization.

21289561 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 27062.

The product of its digits is 8640, while the sum is 34.

The square root of 21289561 is about 4614.0612262951. The cubic root of 21289561 is about 277.1546921015.

The spelling of 21289561 in words is "twenty-one million, two hundred eighty-nine thousand, five hundred sixty-one".