21538409 has 2 divisors, whose sum is σ = 21538410. Its totient is φ = 21538408.

The previous prime is 21538393. The next prime is 21538421. The reversal of 21538409 is 90483512.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 14462809 + 7075600 = 3803^2 + 2660^2 .

It is a cyclic number.

It is not a de Polignac number, because 21538409 - 2⁴ = 21538393 is a prime.

It is a Chen prime.

It is not a weakly prime, because it can be changed into another prime (21538499) 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, 10769204 + 10769205.

It is an arithmetic number, because the mean of its divisors is an integer number (10769205).

Almost surely, 2^21538409 is an apocalyptic number.

It is an amenable number.

21538409 is a deficient number, since it is larger than the sum of its proper divisors (1).

21538409 is an equidigital number, since it uses as much as digits as its factorization.

21538409 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 8640, while the sum is 32.

The square root of 21538409 is about 4640.9491486117. The cubic root of 21538409 is about 278.2303742279.

The spelling of 21538409 in words is "twenty-one million, five hundred thirty-eight thousand, four hundred nine".