21397931 has 2 divisors, whose sum is σ = 21397932. Its totient is φ = 21397930.

The previous prime is 21397919. The next prime is 21397949. The reversal of 21397931 is 13979312.

It is a weak prime.

It is a cyclic number.

It is not a de Polignac number, because 21397931 - 2⁶ = 21397867 is a prime.

It is a super-2 number, since 2×21397931² = 915742902161522, which contains 22 as substring.

It is a junction number, because it is equal to n+sod(n) for n = 21397891 and 21397900.

It is not a weakly prime, because it can be changed into another prime (21397631) 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, 10698965 + 10698966.

It is an arithmetic number, because the mean of its divisors is an integer number (10698966).

Almost surely, 2^21397931 is an apocalyptic number.

21397931 is a deficient number, since it is larger than the sum of its proper divisors (1).

21397931 is an equidigital number, since it uses as much as digits as its factorization.

21397931 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 10206, while the sum is 35.

The square root of 21397931 is about 4625.7897704068. The cubic root of 21397931 is about 277.6241621689.

The spelling of 21397931 in words is "twenty-one million, three hundred ninety-seven thousand, nine hundred thirty-one".