21375821 has 2 divisors, whose sum is σ = 21375822. Its totient is φ = 21375820.

The previous prime is 21375811. The next prime is 21375829. The reversal of 21375821 is 12857312.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 16491721 + 4884100 = 4061^2 + 2210^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21375821 is a prime.

It is a Chen prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21375829) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10687910 + 10687911.

It is an arithmetic number, because the mean of its divisors is an integer number (10687911).

Almost surely, 2^21375821 is an apocalyptic number.

It is an amenable number.

21375821 is a deficient number, since it is larger than the sum of its proper divisors (1).

21375821 is an equidigital number, since it uses as much as digits as its factorization.

21375821 is an evil number, because the sum of its binary digits is even.

The product of its digits is 3360, while the sum is 29.

The square root of 21375821 is about 4623.3992905653. The cubic root of 21375821 is about 277.5285082851.

The spelling of 21375821 in words is "twenty-one million, three hundred seventy-five thousand, eight hundred twenty-one".