21138409 has 2 divisors, whose sum is σ = 21138410. Its totient is φ = 21138408.

The previous prime is 21138407. The next prime is 21138421. The reversal of 21138409 is 90483112.

It is a happy number.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 20903184 + 235225 = 4572^2 + 485^2 .

It is a cyclic number.

It is not a de Polignac number, because 21138409 - 2¹ = 21138407 is a prime.

Together with 21138407, it forms a pair of twin primes.

It is not a weakly prime, because it can be changed into another prime (21138401) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10569204 + 10569205.

It is an arithmetic number, because the mean of its divisors is an integer number (10569205).

Almost surely, 2^21138409 is an apocalyptic number.

It is an amenable number.

21138409 is a deficient number, since it is larger than the sum of its proper divisors (1).

21138409 is an equidigital number, since it uses as much as digits as its factorization.

21138409 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 1728, while the sum is 28.

The square root of 21138409 is about 4597.6525532058. The cubic root of 21138409 is about 276.4972177343.

The spelling of 21138409 in words is "twenty-one million, one hundred thirty-eight thousand, four hundred nine".