21855781 has 2 divisors, whose sum is σ = 21855782. Its totient is φ = 21855780.

The previous prime is 21855737. The next prime is 21855803. The reversal of 21855781 is 18755812.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 11189025 + 10666756 = 3345^2 + 3266^2 .

It is a cyclic number.

It is not a de Polignac number, because 21855781 - 2¹³ = 21847589 is a prime.

It is a super-2 number, since 2×21855781² = 955350326239922, which contains 22 as substring.

It is a self number, because there is not a number n which added to its sum of digits gives 21855781.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21855881) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10927890 + 10927891.

It is an arithmetic number, because the mean of its divisors is an integer number (10927891).

Almost surely, 2^21855781 is an apocalyptic number.

It is an amenable number.

21855781 is a deficient number, since it is larger than the sum of its proper divisors (1).

21855781 is an equidigital number, since it uses as much as digits as its factorization.

21855781 is an evil number, because the sum of its binary digits is even.

The product of its digits is 22400, while the sum is 37.

The square root of 21855781 is about 4675.0166844622. The cubic root of 21855781 is about 279.5903064446.

The spelling of 21855781 in words is "twenty-one million, eight hundred fifty-five thousand, seven hundred eighty-one".