21891781 has 16 divisors (see below), whose sum is σ = 24235200. Its totient is φ = 19692288.

The previous prime is 21891761. The next prime is 21891787. The reversal of 21891781 is 18719812.

It is a cyclic number.

It is not a de Polignac number, because 21891781 - 2⁹ = 21891269 is a prime.

It is a super-2 number, since 2×21891781² = 958500150703922, which contains 22 as substring.

It is a Duffinian number.

It is a self number, because there is not a number n which added to its sum of digits gives 21891781.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21891787) 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 15 ways as a sum of consecutive naturals, for example, 36621 + ... + 37213.

It is an arithmetic number, because the mean of its divisors is an integer number (1514700).

Almost surely, 2^21891781 is an apocalyptic number.

It is an amenable number.

21891781 is a deficient number, since it is larger than the sum of its proper divisors (2343419).

21891781 is a wasteful number, since it uses less digits than its factorization.

21891781 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 708.

The product of its digits is 8064, while the sum is 37.

The square root of 21891781 is about 4678.8653539079. The cubic root of 21891781 is about 279.7437323615.

The spelling of 21891781 in words is "twenty-one million, eight hundred ninety-one thousand, seven hundred eighty-one".