21205781 has 2 divisors, whose sum is σ = 21205782. Its totient is φ = 21205780.

The previous prime is 21205759. The next prime is 21205801. The reversal of 21205781 is 18750212.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 13942756 + 7263025 = 3734^2 + 2695^2 .

It is a cyclic number.

It is not a de Polignac number, because 21205781 - 2¹⁴ = 21189397 is a prime.

It is a super-2 number, since 2×21205781² = 899370295639922, 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 21205781.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21205381) 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 as a sum of consecutive naturals, namely, 10602890 + 10602891.

It is an arithmetic number, because the mean of its divisors is an integer number (10602891).

Almost surely, 2^21205781 is an apocalyptic number.

It is an amenable number.

21205781 is a deficient number, since it is larger than the sum of its proper divisors (1).

21205781 is an equidigital number, since it uses as much as digits as its factorization.

21205781 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 1120, while the sum is 26.

The square root of 21205781 is about 4604.9735069813. The cubic root of 21205781 is about 276.7906553822.

Adding to 21205781 its reverse (18750212), we get a palindrome (39955993).

The spelling of 21205781 in words is "twenty-one million, two hundred five thousand, seven hundred eighty-one".