21757561 has 8 divisors (see below), whose sum is σ = 24894464. Its totient is φ = 18627840.

The previous prime is 21757559. The next prime is 21757579. The reversal of 21757561 is 16575712.

It is a happy number.

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

It is not a de Polignac number, because 21757561 - 2¹ = 21757559 is a prime.

It is the 4665-th Hogben number.

It is not an unprimeable number, because it can be changed into a prime (21757361) by changing a digit.

It is a nontrivial repunit in base 4664.

It is a pernicious number, because its binary representation contains a prime number (17) of ones.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 9241 + ... + 11353.

It is an arithmetic number, because the mean of its divisors is an integer number (3111808).

Almost surely, 2^21757561 is an apocalyptic number.

It is an amenable number.

21757561 is a deficient number, since it is larger than the sum of its proper divisors (3136903).

21757561 is a wasteful number, since it uses less digits than its factorization.

21757561 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 3591.

The product of its digits is 14700, while the sum is 34.

The square root of 21757561 is about 4664.5000803945. The cubic root of 21757561 is about 279.1708506786.

The spelling of 21757561 in words is "twenty-one million, seven hundred fifty-seven thousand, five hundred sixty-one".