21287561 has 4 divisors (see below), whose sum is σ = 21372624. Its totient is φ = 21202500.

The previous prime is 21287551. The next prime is 21287633. The reversal of 21287561 is 16578212.

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 21287561 - 2¹⁰ = 21286537 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21287531) 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, 42155 + ... + 42656.

It is an arithmetic number, because the mean of its divisors is an integer number (5343156).

Almost surely, 2^21287561 is an apocalyptic number.

It is an amenable number.

21287561 is a deficient number, since it is larger than the sum of its proper divisors (85063).

21287561 is an equidigital number, since it uses as much as digits as its factorization.

21287561 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 85062.

The product of its digits is 6720, while the sum is 32.

The square root of 21287561 is about 4613.8444923946. The cubic root of 21287561 is about 277.1460129380.

The spelling of 21287561 in words is "twenty-one million, two hundred eighty-seven thousand, five hundred sixty-one".