21097421 has 2 divisors, whose sum is σ = 21097422. Its totient is φ = 21097420.

The previous prime is 21097393. The next prime is 21097423. The reversal of 21097421 is 12479012.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 20884900 + 212521 = 4570^2 + 461^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21097421 is a prime.

It is a Sophie Germain prime.

Together with 21097423, it forms a pair of twin primes.

It is a Chen prime.

It is a Curzon number.

It is equal to p_1335725 and since 21097421 and 1335725 have the same sum of digits, it is a Honaker prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21097423) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10548710 + 10548711.

It is an arithmetic number, because the mean of its divisors is an integer number (10548711).

Almost surely, 2^21097421 is an apocalyptic number.

It is an amenable number.

21097421 is a deficient number, since it is larger than the sum of its proper divisors (1).

21097421 is an equidigital number, since it uses as much as digits as its factorization.

21097421 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 1008, while the sum is 26.

The square root of 21097421 is about 4593.1928981918. The cubic root of 21097421 is about 276.3183900071.

The spelling of 21097421 in words is "twenty-one million, ninety-seven thousand, four hundred twenty-one".