21751561 has 16 divisors (see below), whose sum is σ = 24978240. Its totient is φ = 18774720.

The previous prime is 21751559. The next prime is 21751567. The reversal of 21751561 is 16515712.

It is a cyclic number.

It is not a de Polignac number, because 21751561 - 2¹ = 21751559 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21751567) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 19971 + ... + 21031.

It is an arithmetic number, because the mean of its divisors is an integer number (1561140).

Almost surely, 2^21751561 is an apocalyptic number.

It is an amenable number.

21751561 is a deficient number, since it is larger than the sum of its proper divisors (3226679).

21751561 is a wasteful number, since it uses less digits than its factorization.

21751561 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1176.

The product of its digits is 2100, while the sum is 28.

The square root of 21751561 is about 4663.8568803084. The cubic root of 21751561 is about 279.1451863575.

The spelling of 21751561 in words is "twenty-one million, seven hundred fifty-one thousand, five hundred sixty-one".