21661751 has 4 divisors (see below), whose sum is σ = 21679992. Its totient is φ = 21643512.

The previous prime is 21661733. The next prime is 21661781. The reversal of 21661751 is 15716612.

It is a semiprime because it is the product of two primes.

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21661751 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21661711) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 7205 + ... + 9758.

It is an arithmetic number, because the mean of its divisors is an integer number (5419998).

Almost surely, 2^21661751 is an apocalyptic number.

21661751 is a deficient number, since it is larger than the sum of its proper divisors (18241).

21661751 is a wasteful number, since it uses less digits than its factorization.

21661751 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 18240.

The product of its digits is 2520, while the sum is 29.

The square root of 21661751 is about 4654.2186240012. The cubic root of 21661751 is about 278.7604689507.

The spelling of 21661751 in words is "twenty-one million, six hundred sixty-one thousand, seven hundred fifty-one".