21781013 has 4 divisors (see below), whose sum is σ = 21791328. Its totient is φ = 21770700.

The previous prime is 21780991. The next prime is 21781021. The reversal of 21781013 is 31018712.

It is a happy number.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4, and also a brilliant number, because the two primes have the same length.

It is a cyclic number.

It is not a de Polignac number, because 21781013 - 2⁶ = 21780949 is a prime.

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 21781013.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21781093) 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, 713 + ... + 6638.

It is an arithmetic number, because the mean of its divisors is an integer number (5447832).

Almost surely, 2^21781013 is an apocalyptic number.

It is an amenable number.

21781013 is a deficient number, since it is larger than the sum of its proper divisors (10315).

21781013 is an equidigital number, since it uses as much as digits as its factorization.

21781013 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 10314.

The product of its (nonzero) digits is 336, while the sum is 23.

The square root of 21781013 is about 4667.0132847465. The cubic root of 21781013 is about 279.2711187167.

Adding to 21781013 its reverse (31018712), we get a palindrome (52799725).

The spelling of 21781013 in words is "twenty-one million, seven hundred eighty-one thousand, thirteen".