21339781 has 2 divisors, whose sum is σ = 21339782. Its totient is φ = 21339780.

The previous prime is 21339779. The next prime is 21339809. The reversal of 21339781 is 18793312.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 12489156 + 8850625 = 3534^2 + 2975^2 .

It is a cyclic number.

It is not a de Polignac number, because 21339781 - 2¹ = 21339779 is a prime.

It is a super-2 number, since 2×21339781² = 910772506255922, which contains 22 as substring.

Together with 21339779, it forms a pair of twin primes.

It is the 4620-th Hogben number.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21339751) by changing a digit.

It is a nontrivial repunit in base 4619.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10669890 + 10669891.

It is an arithmetic number, because the mean of its divisors is an integer number (10669891).

Almost surely, 2^21339781 is an apocalyptic number.

It is an amenable number.

21339781 is a deficient number, since it is larger than the sum of its proper divisors (1).

21339781 is an equidigital number, since it uses as much as digits as its factorization.

21339781 is an evil number, because the sum of its binary digits is even.

The product of its digits is 9072, while the sum is 34.

The square root of 21339781 is about 4619.5000811776. The cubic root of 21339781 is about 277.3724479407.

The spelling of 21339781 in words is "twenty-one million, three hundred thirty-nine thousand, seven hundred eighty-one".