21891561 has 8 divisors (see below), whose sum is σ = 30457920. Its totient is φ = 13959792.

The previous prime is 21891557. The next prime is 21891577. The reversal of 21891561 is 16519812.

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

It is not a de Polignac number, because 21891561 - 2² = 21891557 is a prime.

It is not an unprimeable number, because it can be changed into a prime (21891511) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 158566 + ... + 158703.

It is an arithmetic number, because the mean of its divisors is an integer number (3807240).

Almost surely, 2^21891561 is an apocalyptic number.

It is an amenable number.

21891561 is a deficient number, since it is larger than the sum of its proper divisors (8566359).

21891561 is a wasteful number, since it uses less digits than its factorization.

21891561 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 317295.

The product of its digits is 4320, while the sum is 33.

The square root of 21891561 is about 4678.8418438755. The cubic root of 21891561 is about 279.7427952697.

It can be divided in two parts, 21 and 891561, that multiplied together give a palindrome (18722781).

The spelling of 21891561 in words is "twenty-one million, eight hundred ninety-one thousand, five hundred sixty-one".