21399421 has 2 divisors, whose sum is σ = 21399422. Its totient is φ = 21399420.

The previous prime is 21399401. The next prime is 21399437. The reversal of 21399421 is 12499312.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 11088900 + 10310521 = 3330^2 + 3211^2 .

It is a cyclic number.

It is not a de Polignac number, because 21399421 - 2²³ = 13010813 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21399401) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10699710 + 10699711.

It is an arithmetic number, because the mean of its divisors is an integer number (10699711).

Almost surely, 2^21399421 is an apocalyptic number.

It is an amenable number.

21399421 is a deficient number, since it is larger than the sum of its proper divisors (1).

21399421 is an equidigital number, since it uses as much as digits as its factorization.

21399421 is an evil number, because the sum of its binary digits is even.

The product of its digits is 3888, while the sum is 31.

The square root of 21399421 is about 4625.9508211826. The cubic root of 21399421 is about 277.6306059446.

The spelling of 21399421 in words is "twenty-one million, three hundred ninety-nine thousand, four hundred twenty-one".