21981765 has 64 divisors (see below), whose sum is σ = 42336000. Its totient is φ = 9621504.

The previous prime is 21981763. The next prime is 21981793. The reversal of 21981765 is 56718912.

21981765 is a nontrivial binomial coefficient, being equal to C(6631, 2).

It is not a de Polignac number, because 21981765 - 2¹ = 21981763 is a prime.

It is a Harshad number since it is a multiple of its sum of digits (39).

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21981763) 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 63 ways as a sum of consecutive naturals, for example, 62811 + ... + 63159.

It is an arithmetic number, because the mean of its divisors is an integer number (661500).

Almost surely, 2^21981765 is an apocalyptic number.

21981765 is the 6630-th triangular number.

It is an amenable number.

21981765 is a deficient number, since it is larger than the sum of its proper divisors (20354235).

21981765 is a wasteful number, since it uses less digits than its factorization.

21981765 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 406.

The product of its digits is 30240, while the sum is 39.

The square root of 21981765 is about 4688.4714993268. The cubic root of 21981765 is about 280.1264947160.

The spelling of 21981765 in words is "twenty-one million, nine hundred eighty-one thousand, seven hundred sixty-five".