21397813 has 2 divisors, whose sum is σ = 21397814. Its totient is φ = 21397812.

The previous prime is 21397811. The next prime is 21397823. The reversal of 21397813 is 31879312.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 11675889 + 9721924 = 3417^2 + 3118^2 .

It is a cyclic number.

It is not a de Polignac number, because 21397813 - 2¹ = 21397811 is a prime.

Together with 21397811, it forms a pair of twin primes.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21397811) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10698906 + 10698907.

It is an arithmetic number, because the mean of its divisors is an integer number (10698907).

Almost surely, 2^21397813 is an apocalyptic number.

It is an amenable number.

21397813 is a deficient number, since it is larger than the sum of its proper divisors (1).

21397813 is an equidigital number, since it uses as much as digits as its factorization.

21397813 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 21397813 is about 4625.7770158104. Note that the first 3 decimals coincide. The cubic root of 21397813 is about 277.6236518437.

The spelling of 21397813 in words is "twenty-one million, three hundred ninety-seven thousand, eight hundred thirteen".